| Message ID | alpine.DEB.2.21.1806211434420.51095@chino.kir.corp.google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Ping? This can be something that can easily be removed if it becomes obsoleted because the oom reaper is always able to free memory to the extent of exit_mmap(). I argue that it cannot, because it cannot do free_pgtables() for large amounts of virtual memory, but am fine to be proved wrong. In the meantime, however, this patch should introduce no significant change in functionality and the only interface it is added is in debugfs and can easily be removed if it is obsoleted. The work to make the oom reaper more effective or realible can still continue with this patch. On Thu, 21 Jun 2018, David Rientjes wrote: > The oom reaper ensures forward progress by setting MMF_OOM_SKIP itself if > it cannot reap an mm. This can happen for a variety of reasons, > including: > > - the inability to grab mm->mmap_sem in a sufficient amount of time, > > - when the mm has blockable mmu notifiers that could cause the oom reaper > to stall indefinitely, > > but we can also add a third when the oom reaper can "reap" an mm but doing > so is unlikely to free any amount of memory: > > - when the mm's memory is mostly mlocked. > > When all memory is mlocked, the oom reaper will not be able to free any > substantial amount of memory. It sets MMF_OOM_SKIP before the victim can > unmap and free its memory in exit_mmap() and subsequent oom victims are > chosen unnecessarily. This is trivial to reproduce if all eligible > processes on the system have mlocked their memory: the oom killer calls > panic() even though forward progress can be made. > > This is the same issue where the exit path sets MMF_OOM_SKIP before > unmapping memory and additional processes can be chosen unnecessarily > because the oom killer is racing with exit_mmap() and is separate from > the oom reaper setting MMF_OOM_SKIP prematurely. > > We can't simply defer setting MMF_OOM_SKIP, however, because if there is > a true oom livelock in progress, it never gets set and no additional > killing is possible. > > To fix this, this patch introduces a per-mm reaping period, which is > configurable through the new oom_free_timeout_ms file in debugfs and > defaults to one second to match the current heuristics. This support > requires that the oom reaper's list becomes a proper linked list so that > other mm's may be reaped while waiting for an mm's timeout to expire. > > This replaces the current timeouts in the oom reaper: (1) when trying to > grab mm->mmap_sem 10 times in a row with HZ/10 sleeps in between and (2) > a HZ sleep if there are blockable mmu notifiers. It extends it with > timeout to allow an oom victim to reach exit_mmap() before choosing > additional processes unnecessarily. > > The exit path will now set MMF_OOM_SKIP only after all memory has been > freed, so additional oom killing is justified, and rely on MMF_UNSTABLE to > determine when it can race with the oom reaper. > > The oom reaper will now set MMF_OOM_SKIP only after the reap timeout has > lapsed because it can no longer guarantee forward progress. Since the > default oom_free_timeout_ms is one second, the same as current heuristics, > there should be no functional change with this patch for users who do not > tune it to be longer other than MMF_OOM_SKIP is set by exit_mmap() after > free_pgtables(), which is the preferred behavior. > > The reaping timeout can intentionally be set for a substantial amount of > time, such as 10s, since oom livelock is a very rare occurrence and it's > better to optimize for preventing additional (unnecessary) oom killing > than a scenario that is much more unlikely. > > Signed-off-by: David Rientjes <rientjes@google.com> > --- > v3: > - oom_free_timeout_ms is now static per kbuild test robot > > v2: > - configurable timeout period through debugfs > - change mm->reap_timeout to mm->oom_free_expire and add more > descriptive comment per akpm > - add comment to describe task->oom_reap_list locking based on > oom_reaper_lock per akpm > - rework the exit_mmap() comment and split into two parts to be more > descriptive about the locking and the issue with the oom reaper > racing with munlock_vma_pages_all() per akpm > --- > include/linux/mm_types.h | 7 ++ > include/linux/sched.h | 3 +- > kernel/fork.c | 3 + > mm/mmap.c | 26 +++++--- > mm/oom_kill.c | 140 +++++++++++++++++++++++++-------------- > 5 files changed, 119 insertions(+), 60 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -449,6 +449,13 @@ struct mm_struct { > #ifdef CONFIG_MMU_NOTIFIER > struct mmu_notifier_mm *mmu_notifier_mm; > #endif > +#ifdef CONFIG_MMU > + /* > + * When to give up on memory freeing from this mm after its > + * threads have been oom killed, in jiffies. > + */ > + unsigned long oom_free_expire; > +#endif > #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS > pgtable_t pmd_huge_pte; /* protected by page_table_lock */ > #endif > diff --git a/include/linux/sched.h b/include/linux/sched.h > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1163,7 +1163,8 @@ struct task_struct { > #endif > int pagefault_disabled; > #ifdef CONFIG_MMU > - struct task_struct *oom_reaper_list; > + /* OOM victim queue for oom reaper, protected by oom_reaper_lock */ > + struct list_head oom_reap_list; > #endif > #ifdef CONFIG_VMAP_STACK > struct vm_struct *stack_vm_area; > diff --git a/kernel/fork.c b/kernel/fork.c > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -842,6 +842,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) > #ifdef CONFIG_FAULT_INJECTION > tsk->fail_nth = 0; > #endif > +#ifdef CONFIG_MMU > + INIT_LIST_HEAD(&tsk->oom_reap_list); > +#endif > > return tsk; > > diff --git a/mm/mmap.c b/mm/mmap.c > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3059,25 +3059,28 @@ void exit_mmap(struct mm_struct *mm) > if (unlikely(mm_is_oom_victim(mm))) { > /* > * Manually reap the mm to free as much memory as possible. > - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard > - * this mm from further consideration. Taking mm->mmap_sem for > - * write after setting MMF_OOM_SKIP will guarantee that the oom > - * reaper will not run on this mm again after mmap_sem is > - * dropped. > - * > * Nothing can be holding mm->mmap_sem here and the above call > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > * __oom_reap_task_mm() will not block. > - * > - * This needs to be done before calling munlock_vma_pages_all(), > - * which clears VM_LOCKED, otherwise the oom reaper cannot > - * reliably test it. > */ > mutex_lock(&oom_lock); > __oom_reap_task_mm(mm); > mutex_unlock(&oom_lock); > > - set_bit(MMF_OOM_SKIP, &mm->flags); > + /* > + * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. > + * This needs to be done before calling munlock_vma_pages_all(), > + * which clears VM_LOCKED, otherwise the oom reaper cannot > + * reliably test for it. If the oom reaper races with > + * munlock_vma_pages_all(), this can result in a kernel oops if > + * a pmd is zapped, for example, after follow_page_mask() has > + * checked pmd_none(). > + * > + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will > + * guarantee that the oom reaper will not run on this mm again > + * after mmap_sem is dropped. > + */ > + set_bit(MMF_UNSTABLE, &mm->flags); > down_write(&mm->mmap_sem); > up_write(&mm->mmap_sem); > } > @@ -3105,6 +3108,7 @@ void exit_mmap(struct mm_struct *mm) > unmap_vmas(&tlb, vma, 0, -1); > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); > tlb_finish_mmu(&tlb, 0, -1); > + set_bit(MMF_OOM_SKIP, &mm->flags); > > /* > * Walk the list again, actually closing and freeing it, > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -41,6 +41,7 @@ > #include <linux/kthread.h> > #include <linux/init.h> > #include <linux/mmu_notifier.h> > +#include <linux/debugfs.h> > > #include <asm/tlb.h> > #include "internal.h" > @@ -476,7 +477,7 @@ bool process_shares_mm(struct task_struct *p, struct mm_struct *mm) > */ > static struct task_struct *oom_reaper_th; > static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait); > -static struct task_struct *oom_reaper_list; > +static LIST_HEAD(oom_reaper_list); > static DEFINE_SPINLOCK(oom_reaper_lock); > > void __oom_reap_task_mm(struct mm_struct *mm) > @@ -519,10 +520,8 @@ void __oom_reap_task_mm(struct mm_struct *mm) > } > } > > -static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > +static void oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > { > - bool ret = true; > - > /* > * We have to make sure to not race with the victim exit path > * and cause premature new oom victim selection: > @@ -540,9 +539,8 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > mutex_lock(&oom_lock); > > if (!down_read_trylock(&mm->mmap_sem)) { > - ret = false; > trace_skip_task_reaping(tsk->pid); > - goto unlock_oom; > + goto out_oom; > } > > /* > @@ -551,69 +549,81 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) > * TODO: we really want to get rid of this ugly hack and make sure that > * notifiers cannot block for unbounded amount of time > */ > - if (mm_has_blockable_invalidate_notifiers(mm)) { > - up_read(&mm->mmap_sem); > - schedule_timeout_idle(HZ); > - goto unlock_oom; > - } > + if (mm_has_blockable_invalidate_notifiers(mm)) > + goto out_mm; > > /* > - * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't > - * work on the mm anymore. The check for MMF_OOM_SKIP must run > + * MMF_UNSTABLE is set by exit_mmap when the OOM reaper can't > + * work on the mm anymore. The check for MMF_UNSTABLE must run > * under mmap_sem for reading because it serializes against the > * down_write();up_write() cycle in exit_mmap(). > */ > - if (test_bit(MMF_OOM_SKIP, &mm->flags)) { > - up_read(&mm->mmap_sem); > + if (test_bit(MMF_UNSTABLE, &mm->flags)) { > trace_skip_task_reaping(tsk->pid); > - goto unlock_oom; > + goto out_mm; > } > > trace_start_task_reaping(tsk->pid); > - > __oom_reap_task_mm(mm); > + trace_finish_task_reaping(tsk->pid); > > pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n", > task_pid_nr(tsk), tsk->comm, > K(get_mm_counter(mm, MM_ANONPAGES)), > K(get_mm_counter(mm, MM_FILEPAGES)), > K(get_mm_counter(mm, MM_SHMEMPAGES))); > +out_mm: > up_read(&mm->mmap_sem); > - > - trace_finish_task_reaping(tsk->pid); > -unlock_oom: > +out_oom: > mutex_unlock(&oom_lock); > - return ret; > } > > -#define MAX_OOM_REAP_RETRIES 10 > static void oom_reap_task(struct task_struct *tsk) > { > - int attempts = 0; > struct mm_struct *mm = tsk->signal->oom_mm; > > - /* Retry the down_read_trylock(mmap_sem) a few times */ > - while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm)) > - schedule_timeout_idle(HZ/10); > + /* > + * If this mm has either been fully unmapped, or the oom reaper has > + * given up on it, nothing left to do except drop the refcount. > + */ > + if (test_bit(MMF_OOM_SKIP, &mm->flags)) > + goto drop; > > - if (attempts <= MAX_OOM_REAP_RETRIES || > - test_bit(MMF_OOM_SKIP, &mm->flags)) > - goto done; > + /* > + * If this mm has already been reaped, doing so again will not likely > + * free additional memory. > + */ > + if (!test_bit(MMF_UNSTABLE, &mm->flags)) > + oom_reap_task_mm(tsk, mm); > > - pr_info("oom_reaper: unable to reap pid:%d (%s)\n", > - task_pid_nr(tsk), tsk->comm); > - debug_show_all_locks(); > + if (time_after_eq(jiffies, mm->oom_free_expire)) { > + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) { > + pr_info("oom_reaper: unable to reap pid:%d (%s)\n", > + task_pid_nr(tsk), tsk->comm); > + debug_show_all_locks(); > > -done: > - tsk->oom_reaper_list = NULL; > + /* > + * Reaping has failed for the timeout period, so give up > + * and allow additional processes to be oom killed. > + */ > + set_bit(MMF_OOM_SKIP, &mm->flags); > + } > + goto drop; > + } > > - /* > - * Hide this mm from OOM killer because it has been either reaped or > - * somebody can't call up_write(mmap_sem). > - */ > - set_bit(MMF_OOM_SKIP, &mm->flags); > + if (test_bit(MMF_OOM_SKIP, &mm->flags)) > + goto drop; > + > + /* Enqueue to be reaped again */ > + spin_lock(&oom_reaper_lock); > + list_add_tail(&tsk->oom_reap_list, &oom_reaper_list); > + spin_unlock(&oom_reaper_lock); > > - /* Drop a reference taken by wake_oom_reaper */ > + schedule_timeout_idle(HZ/10); > + return; > + > +drop: > + /* Drop the reference taken by wake_oom_reaper */ > put_task_struct(tsk); > } > > @@ -622,11 +632,13 @@ static int oom_reaper(void *unused) > while (true) { > struct task_struct *tsk = NULL; > > - wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL); > + wait_event_freezable(oom_reaper_wait, > + !list_empty(&oom_reaper_list)); > spin_lock(&oom_reaper_lock); > - if (oom_reaper_list != NULL) { > - tsk = oom_reaper_list; > - oom_reaper_list = tsk->oom_reaper_list; > + if (!list_empty(&oom_reaper_list)) { > + tsk = list_entry(oom_reaper_list.next, > + struct task_struct, oom_reap_list); > + list_del(&tsk->oom_reap_list); > } > spin_unlock(&oom_reaper_lock); > > @@ -637,25 +649,57 @@ static int oom_reaper(void *unused) > return 0; > } > > +/* > + * Millisecs to wait for an oom mm to free memory before selecting another > + * victim. > + */ > +static u64 oom_free_timeout_ms = 1000; > static void wake_oom_reaper(struct task_struct *tsk) > { > - /* tsk is already queued? */ > - if (tsk == oom_reaper_list || tsk->oom_reaper_list) > + /* > + * Set the reap timeout; if it's already set, the mm is enqueued and > + * this tsk can be ignored. > + */ > + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, > + jiffies + msecs_to_jiffies(oom_free_timeout_ms))) > return; > > get_task_struct(tsk); > > spin_lock(&oom_reaper_lock); > - tsk->oom_reaper_list = oom_reaper_list; > - oom_reaper_list = tsk; > + list_add(&tsk->oom_reap_list, &oom_reaper_list); > spin_unlock(&oom_reaper_lock); > trace_wake_reaper(tsk->pid); > wake_up(&oom_reaper_wait); > } > > +#ifdef CONFIG_DEBUG_FS > +static int oom_free_timeout_ms_read(void *data, u64 *val) > +{ > + *val = oom_free_timeout_ms; > + return 0; > +} > + > +static int oom_free_timeout_ms_write(void *data, u64 val) > +{ > + if (val > 60 * 1000) > + return -EINVAL; > + > + oom_free_timeout_ms = val; > + return 0; > +} > +DEFINE_SIMPLE_ATTRIBUTE(oom_free_timeout_ms_fops, oom_free_timeout_ms_read, > + oom_free_timeout_ms_write, "%llu\n"); > +#endif /* CONFIG_DEBUG_FS */ > + > static int __init oom_init(void) > { > oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper"); > +#ifdef CONFIG_DEBUG_FS > + if (!IS_ERR(oom_reaper_th)) > + debugfs_create_file("oom_free_timeout_ms", 0200, NULL, NULL, > + &oom_free_timeout_ms_fops); > +#endif > return 0; > } > subsys_initcall(oom_init) >
David Rientjes wrote: > Ping? > > This can be something that can easily be removed if it becomes obsoleted > because the oom reaper is always able to free memory to the extent of > exit_mmap(). I argue that it cannot, because it cannot do free_pgtables() > for large amounts of virtual memory, but am fine to be proved wrong. This is "[PATCH 3/8] mm,oom: Fix unnecessary killing of additional processes." in my series. > > In the meantime, however, this patch should introduce no significant > change in functionality and the only interface it is added is in debugfs > and can easily be removed if it is obsoleted. > > The work to make the oom reaper more effective or realible can still > continue with this patch. >
On Thu, 21 Jun 2018 14:35:20 -0700 (PDT) David Rientjes <rientjes@google.com> wrote: > The oom reaper ensures forward progress by setting MMF_OOM_SKIP itself if > it cannot reap an mm. This can happen for a variety of reasons, > including: > > - the inability to grab mm->mmap_sem in a sufficient amount of time, > > - when the mm has blockable mmu notifiers that could cause the oom reaper > to stall indefinitely, > > but we can also add a third when the oom reaper can "reap" an mm but doing > so is unlikely to free any amount of memory: > > - when the mm's memory is mostly mlocked. Michal has been talking about making the oom-reaper handle mlocked memory. Where are we at with that? > When all memory is mlocked, the oom reaper will not be able to free any > substantial amount of memory. It sets MMF_OOM_SKIP before the victim can > unmap and free its memory in exit_mmap() and subsequent oom victims are > chosen unnecessarily. This is trivial to reproduce if all eligible > processes on the system have mlocked their memory: the oom killer calls > panic() even though forward progress can be made. > > This is the same issue where the exit path sets MMF_OOM_SKIP before > unmapping memory and additional processes can be chosen unnecessarily > because the oom killer is racing with exit_mmap() and is separate from > the oom reaper setting MMF_OOM_SKIP prematurely. > > We can't simply defer setting MMF_OOM_SKIP, however, because if there is > a true oom livelock in progress, it never gets set and no additional > killing is possible. > > To fix this, this patch introduces a per-mm reaping period, which is > configurable through the new oom_free_timeout_ms file in debugfs and > defaults to one second to match the current heuristics. This support > requires that the oom reaper's list becomes a proper linked list so that > other mm's may be reaped while waiting for an mm's timeout to expire. > > This replaces the current timeouts in the oom reaper: (1) when trying to > grab mm->mmap_sem 10 times in a row with HZ/10 sleeps in between and (2) > a HZ sleep if there are blockable mmu notifiers. It extends it with > timeout to allow an oom victim to reach exit_mmap() before choosing > additional processes unnecessarily. > > The exit path will now set MMF_OOM_SKIP only after all memory has been > freed, so additional oom killing is justified, and rely on MMF_UNSTABLE to > determine when it can race with the oom reaper. > > The oom reaper will now set MMF_OOM_SKIP only after the reap timeout has > lapsed because it can no longer guarantee forward progress. Since the > default oom_free_timeout_ms is one second, the same as current heuristics, > there should be no functional change with this patch for users who do not > tune it to be longer other than MMF_OOM_SKIP is set by exit_mmap() after > free_pgtables(), which is the preferred behavior. > > The reaping timeout can intentionally be set for a substantial amount of > time, such as 10s, since oom livelock is a very rare occurrence and it's > better to optimize for preventing additional (unnecessary) oom killing > than a scenario that is much more unlikely. > > .. > > +#ifdef CONFIG_DEBUG_FS > +static int oom_free_timeout_ms_read(void *data, u64 *val) > +{ > + *val = oom_free_timeout_ms; > + return 0; > +} > + > +static int oom_free_timeout_ms_write(void *data, u64 val) > +{ > + if (val > 60 * 1000) > + return -EINVAL; > + > + oom_free_timeout_ms = val; > + return 0; > +} > +DEFINE_SIMPLE_ATTRIBUTE(oom_free_timeout_ms_fops, oom_free_timeout_ms_read, > + oom_free_timeout_ms_write, "%llu\n"); > +#endif /* CONFIG_DEBUG_FS */ One of the several things I dislike about debugfs is that nobody bothers documenting it anywhere. But this should really be documented. I'm not sure where, but the documentation will find itself alongside a bunch of procfs things which prompts the question "why it *this* one in debugfs"? > static int __init oom_init(void) > { > oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper"); > +#ifdef CONFIG_DEBUG_FS > + if (!IS_ERR(oom_reaper_th)) > + debugfs_create_file("oom_free_timeout_ms", 0200, NULL, NULL, > + &oom_free_timeout_ms_fops); > +#endif > return 0; > } > subsys_initcall(oom_init)
On Thu 05-07-18 16:46:21, Andrew Morton wrote: > On Thu, 21 Jun 2018 14:35:20 -0700 (PDT) David Rientjes <rientjes@google.com> wrote: > > > The oom reaper ensures forward progress by setting MMF_OOM_SKIP itself if > > it cannot reap an mm. This can happen for a variety of reasons, > > including: > > > > - the inability to grab mm->mmap_sem in a sufficient amount of time, > > > > - when the mm has blockable mmu notifiers that could cause the oom reaper > > to stall indefinitely, > > > > but we can also add a third when the oom reaper can "reap" an mm but doing > > so is unlikely to free any amount of memory: > > > > - when the mm's memory is mostly mlocked. > > Michal has been talking about making the oom-reaper handle mlocked > memory. Where are we at with that? I didn't get to mlocked memory yet because blockable mmu notifiers are more important. And I've already posted patch for that and it is under discussion [1]. Mlocked memory is next. [1] http://lkml.kernel.org/r/20180627074421.GF32348@dhcp22.suse.cz Btw. I still hate this patch and making any timeout user defineable. It is a wrong approach and my nack to this patch still applies.
On Thu, 5 Jul 2018, Andrew Morton wrote: > > +#ifdef CONFIG_DEBUG_FS > > +static int oom_free_timeout_ms_read(void *data, u64 *val) > > +{ > > + *val = oom_free_timeout_ms; > > + return 0; > > +} > > + > > +static int oom_free_timeout_ms_write(void *data, u64 val) > > +{ > > + if (val > 60 * 1000) > > + return -EINVAL; > > + > > + oom_free_timeout_ms = val; > > + return 0; > > +} > > +DEFINE_SIMPLE_ATTRIBUTE(oom_free_timeout_ms_fops, oom_free_timeout_ms_read, > > + oom_free_timeout_ms_write, "%llu\n"); > > +#endif /* CONFIG_DEBUG_FS */ > > One of the several things I dislike about debugfs is that nobody > bothers documenting it anywhere. But this should really be documented. > I'm not sure where, but the documentation will find itself alongside a > bunch of procfs things which prompts the question "why it *this* one in > debugfs"? > The only reason I have placed it in debugfs, or making it tunable at all, is to appease others. I know the non-default value we need to use to stop millions of processes being oom killed unnecessarily. Michal suggested a tunable to disable the oom reaper entirely, which is not what we want, so I found this to be the best alternative. I'd like to say that it is purposefully undocumented since it's not a sysctl and nobody can suggest that it is becoming a permanent API that we must maintain for backwards compatibility. Having it be configurable is kind of ridiculous, but such is the nature of trying to get patches merged these days to prevent millions of processes being oom killed unnecessarily. Blockable mmu notifiers and mlocked memory is not the extent of the problem, if a process has a lot of virtual memory we must wait until free_pgtables() completes in exit_mmap() to prevent unnecessary oom killing. For implementations such as tcmalloc, which does not release virtual memory, this is important because, well, it releases this only at exit_mmap(). Of course we cannot do that with only the protection of mm->mmap_sem for read. This is a patch that we'll always need if we continue with the current implementation of the oom reaper. I wouldn't suggest it as a configurable value, but, owell. I'll document the tunable and purposefully repeat myself that this is addresses millions of processes being oom killed unnecessarily so the rather important motivation of the change is clear to anyone who reads this thread now or in the future. Nobody can guess an appropriate value until they have been hit by the issue themselves and need to deal with the loss of work from important processes being oom killed when some best effort logging cron job uses too much memory. Or, of course, pissed off users who have their jobs killed off and you find yourself in the rather unfortunate situation of explaining why the Linux kernel in 2018 needs to immediately SIGKILL processes because of an arbitrary nack related to a timestamp. Thanks.
On Fri 06-07-18 17:05:39, David Rientjes wrote: [...] > Blockable mmu notifiers and mlocked memory is not the extent of the > problem, if a process has a lot of virtual memory we must wait until > free_pgtables() completes in exit_mmap() to prevent unnecessary oom > killing. For implementations such as tcmalloc, which does not release > virtual memory, this is important because, well, it releases this only at > exit_mmap(). Of course we cannot do that with only the protection of > mm->mmap_sem for read. And how exactly a timeout helps to prevent from "unnecessary killing" in that case?
On Mon, 9 Jul 2018, Michal Hocko wrote: > > Blockable mmu notifiers and mlocked memory is not the extent of the > > problem, if a process has a lot of virtual memory we must wait until > > free_pgtables() completes in exit_mmap() to prevent unnecessary oom > > killing. For implementations such as tcmalloc, which does not release > > virtual memory, this is important because, well, it releases this only at > > exit_mmap(). Of course we cannot do that with only the protection of > > mm->mmap_sem for read. > > And how exactly a timeout helps to prevent from "unnecessary killing" in > that case? As my patch does, it becomes mandatory to move MMF_OOM_SKIP to after free_pgtables() in exit_mmap() and then repurpose MMF_UNSTABLE to indicate that the oom reaper should not operate on a given mm. In the event we cannot reach MMF_OOM_SKIP, we need to ensure forward progress and that is possible with a timeout period in the very rare instance where additional memory freeing is needed, and without unnecessary oom killing when it is not needed.
On Mon 09-07-18 13:30:10, David Rientjes wrote: > On Mon, 9 Jul 2018, Michal Hocko wrote: > > > > Blockable mmu notifiers and mlocked memory is not the extent of the > > > problem, if a process has a lot of virtual memory we must wait until > > > free_pgtables() completes in exit_mmap() to prevent unnecessary oom > > > killing. For implementations such as tcmalloc, which does not release > > > virtual memory, this is important because, well, it releases this only at > > > exit_mmap(). Of course we cannot do that with only the protection of > > > mm->mmap_sem for read. > > > > And how exactly a timeout helps to prevent from "unnecessary killing" in > > that case? > > As my patch does, it becomes mandatory to move MMF_OOM_SKIP to after > free_pgtables() in exit_mmap() and then repurpose MMF_UNSTABLE to > indicate that the oom reaper should not operate on a given mm. In the > event we cannot reach MMF_OOM_SKIP, we need to ensure forward progress and > that is possible with a timeout period in the very rare instance where > additional memory freeing is needed, and without unnecessary oom killing > when it is not needed. But such a timeout doesn't really know how much to wait so it is more a hack than anything else. The only reason why we set MMF_OOM_SKIP so early in the exit path now is inability to reap mlocked memory. That is something fundamentally solvable. In fact we can really postpone MMF_OOM_SKIP to after free_pgtables. It would require to extend the current handover between the oom reaper and the exit path but it is doable AFAICS. Only the exit path can call free_pgtables but the oom reaper doesn't have to set MMF_OOM_SKIP if it _knows_ that the exit_mmap is already past any point of blocking. Btw, I am quite surprise you are now worried about oom victims with basically no memory mapped and a huge amount of memory in page tables. We have never handled that case properly IIRC. So oom_reaper hasn't added anything new here. That being said, I haven't heard any bug reports for over eager oom killer just because of the oom reaper except your rather non-specific claims about millions of pointless oom invocations. So I am not really convinced we have to rush into a solution. I would much rather work on a proper and comprehensible solution than put one band aid over another. This has been the case in the oom proper for many years and we have ended up with a subtle code which is way too easy to break and nightmare to maintain. Let's not repeat that again please. So do not rush into first idea and let's do the proper development here. This means the proper analysis of the problem, find a solution space and chose one which is the most reasonable long term.
This patch should be dropped from linux-next because it is incorrectly using MMF_UNSTABLE. On 2018/06/22 6:35, David Rientjes wrote: > diff --git a/mm/mmap.c b/mm/mmap.c > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -3059,25 +3059,28 @@ void exit_mmap(struct mm_struct *mm) > if (unlikely(mm_is_oom_victim(mm))) { > /* > * Manually reap the mm to free as much memory as possible. > - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard > - * this mm from further consideration. Taking mm->mmap_sem for > - * write after setting MMF_OOM_SKIP will guarantee that the oom > - * reaper will not run on this mm again after mmap_sem is > - * dropped. > - * > * Nothing can be holding mm->mmap_sem here and the above call > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > * __oom_reap_task_mm() will not block. > - * > - * This needs to be done before calling munlock_vma_pages_all(), > - * which clears VM_LOCKED, otherwise the oom reaper cannot > - * reliably test it. > */ > mutex_lock(&oom_lock); > __oom_reap_task_mm(mm); > mutex_unlock(&oom_lock); > > - set_bit(MMF_OOM_SKIP, &mm->flags); > + /* > + * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. > + * This needs to be done before calling munlock_vma_pages_all(), > + * which clears VM_LOCKED, otherwise the oom reaper cannot > + * reliably test for it. If the oom reaper races with > + * munlock_vma_pages_all(), this can result in a kernel oops if > + * a pmd is zapped, for example, after follow_page_mask() has > + * checked pmd_none(). > + * > + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will > + * guarantee that the oom reaper will not run on this mm again > + * after mmap_sem is dropped. > + */ > + set_bit(MMF_UNSTABLE, &mm->flags); Since MMF_UNSTABLE is set by __oom_reap_task_mm() from exit_mmap() before start reaping (because the purpose of MMF_UNSTABLE is to "tell all users of get_user/copy_from_user etc... that the content is no longer stable"), it cannot be used for a flag for indicating that the OOM reaper can't work on the mm anymore. If the oom_lock serialization is removed, the OOM reaper will give up after (by default) 1 second even if current thread is immediately after set_bit(MMF_UNSTABLE, &mm->flags) from __oom_reap_task_mm() from exit_mmap(). Thus, this patch and the other patch which removes oom_lock serialization should be dropped. > down_write(&mm->mmap_sem); > up_write(&mm->mmap_sem); > } > @@ -637,25 +649,57 @@ static int oom_reaper(void *unused) > return 0; > } > > +/* > + * Millisecs to wait for an oom mm to free memory before selecting another > + * victim. > + */ > +static u64 oom_free_timeout_ms = 1000; > static void wake_oom_reaper(struct task_struct *tsk) > { > - /* tsk is already queued? */ > - if (tsk == oom_reaper_list || tsk->oom_reaper_list) > + /* > + * Set the reap timeout; if it's already set, the mm is enqueued and > + * this tsk can be ignored. > + */ > + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, > + jiffies + msecs_to_jiffies(oom_free_timeout_ms))) > return; "expire" must not be 0 in order to avoid double list_add(). See https://lore.kernel.org/lkml/201807130620.w6D6KiAJ093010@www262.sakura.ne.jp/T/#u . > > get_task_struct(tsk); > > spin_lock(&oom_reaper_lock); > - tsk->oom_reaper_list = oom_reaper_list; > - oom_reaper_list = tsk; > + list_add(&tsk->oom_reap_list, &oom_reaper_list); > spin_unlock(&oom_reaper_lock); > trace_wake_reaper(tsk->pid); > wake_up(&oom_reaper_wait); > }
On Wed, 18 Jul 2018, Tetsuo Handa wrote: > > diff --git a/mm/mmap.c b/mm/mmap.c > > --- a/mm/mmap.c > > +++ b/mm/mmap.c > > @@ -3059,25 +3059,28 @@ void exit_mmap(struct mm_struct *mm) > > if (unlikely(mm_is_oom_victim(mm))) { > > /* > > * Manually reap the mm to free as much memory as possible. > > - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard > > - * this mm from further consideration. Taking mm->mmap_sem for > > - * write after setting MMF_OOM_SKIP will guarantee that the oom > > - * reaper will not run on this mm again after mmap_sem is > > - * dropped. > > - * > > * Nothing can be holding mm->mmap_sem here and the above call > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > * __oom_reap_task_mm() will not block. > > - * > > - * This needs to be done before calling munlock_vma_pages_all(), > > - * which clears VM_LOCKED, otherwise the oom reaper cannot > > - * reliably test it. > > */ > > mutex_lock(&oom_lock); > > __oom_reap_task_mm(mm); > > mutex_unlock(&oom_lock); > > > > - set_bit(MMF_OOM_SKIP, &mm->flags); > > + /* > > + * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. > > + * This needs to be done before calling munlock_vma_pages_all(), > > + * which clears VM_LOCKED, otherwise the oom reaper cannot > > + * reliably test for it. If the oom reaper races with > > + * munlock_vma_pages_all(), this can result in a kernel oops if > > + * a pmd is zapped, for example, after follow_page_mask() has > > + * checked pmd_none(). > > + * > > + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will > > + * guarantee that the oom reaper will not run on this mm again > > + * after mmap_sem is dropped. > > + */ > > + set_bit(MMF_UNSTABLE, &mm->flags); > > Since MMF_UNSTABLE is set by __oom_reap_task_mm() from exit_mmap() before start reaping > (because the purpose of MMF_UNSTABLE is to "tell all users of get_user/copy_from_user > etc... that the content is no longer stable"), it cannot be used for a flag for indicating > that the OOM reaper can't work on the mm anymore. > Why? It should be able to be set by exit_mmap() since nothing else should be accessing this mm in the first place. There is no reason to wait for the oom reaper and the following down_write();up_write(); cycle will guarantee it is not operating on the mm before munlocking. > If the oom_lock serialization is removed, the OOM reaper will give up after (by default) > 1 second even if current thread is immediately after set_bit(MMF_UNSTABLE, &mm->flags) from > __oom_reap_task_mm() from exit_mmap(). Thus, this patch and the other patch which removes > oom_lock serialization should be dropped. > No, it shouldn't, lol. The oom reaper may give up because we have entered __oom_reap_task_mm() by way of exit_mmap(), there's no other purpose for it acting on the mm. This is very different from giving up by setting MMF_OOM_SKIP, which it will wait for oom_free_timeout_ms to do unless the thread can make forward progress here in exit_mmap(). > > down_write(&mm->mmap_sem); > > up_write(&mm->mmap_sem); > > } > > > @@ -637,25 +649,57 @@ static int oom_reaper(void *unused) > > return 0; > > } > > > > +/* > > + * Millisecs to wait for an oom mm to free memory before selecting another > > + * victim. > > + */ > > +static u64 oom_free_timeout_ms = 1000; > > static void wake_oom_reaper(struct task_struct *tsk) > > { > > - /* tsk is already queued? */ > > - if (tsk == oom_reaper_list || tsk->oom_reaper_list) > > + /* > > + * Set the reap timeout; if it's already set, the mm is enqueued and > > + * this tsk can be ignored. > > + */ > > + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, > > + jiffies + msecs_to_jiffies(oom_free_timeout_ms))) > > return; > > "expire" must not be 0 in order to avoid double list_add(). See > https://lore.kernel.org/lkml/201807130620.w6D6KiAJ093010@www262.sakura.ne.jp/T/#u . > We should not allow oom_free_timeout_ms to be 0 for sure, I assume 1000 is the sane minimum since we need to allow time for some memory freeing and this will not be radically different from what existed before the patch for the various backoffs. Or maybe you meant something else for "expire" here?
Sigh... Nacked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> because David is not aware what is wrong. On 2018/07/19 5:22, David Rientjes wrote: > On Wed, 18 Jul 2018, Tetsuo Handa wrote: > >>> diff --git a/mm/mmap.c b/mm/mmap.c >>> --- a/mm/mmap.c >>> +++ b/mm/mmap.c >>> @@ -3059,25 +3059,28 @@ void exit_mmap(struct mm_struct *mm) >>> if (unlikely(mm_is_oom_victim(mm))) { >>> /* >>> * Manually reap the mm to free as much memory as possible. >>> - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard >>> - * this mm from further consideration. Taking mm->mmap_sem for >>> - * write after setting MMF_OOM_SKIP will guarantee that the oom >>> - * reaper will not run on this mm again after mmap_sem is >>> - * dropped. >>> - * >>> * Nothing can be holding mm->mmap_sem here and the above call >>> * to mmu_notifier_release(mm) ensures mmu notifier callbacks in >>> * __oom_reap_task_mm() will not block. >>> - * >>> - * This needs to be done before calling munlock_vma_pages_all(), >>> - * which clears VM_LOCKED, otherwise the oom reaper cannot >>> - * reliably test it. >>> */ >>> mutex_lock(&oom_lock); >>> __oom_reap_task_mm(mm); >>> mutex_unlock(&oom_lock); >>> >>> - set_bit(MMF_OOM_SKIP, &mm->flags); >>> + /* >>> + * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. >>> + * This needs to be done before calling munlock_vma_pages_all(), >>> + * which clears VM_LOCKED, otherwise the oom reaper cannot >>> + * reliably test for it. If the oom reaper races with >>> + * munlock_vma_pages_all(), this can result in a kernel oops if >>> + * a pmd is zapped, for example, after follow_page_mask() has >>> + * checked pmd_none(). >>> + * >>> + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will >>> + * guarantee that the oom reaper will not run on this mm again >>> + * after mmap_sem is dropped. >>> + */ >>> + set_bit(MMF_UNSTABLE, &mm->flags); >> >> Since MMF_UNSTABLE is set by __oom_reap_task_mm() from exit_mmap() before start reaping >> (because the purpose of MMF_UNSTABLE is to "tell all users of get_user/copy_from_user >> etc... that the content is no longer stable"), it cannot be used for a flag for indicating >> that the OOM reaper can't work on the mm anymore. >> > > Why? It should be able to be set by exit_mmap() since nothing else should > be accessing this mm in the first place. There is no reason to wait for > the oom reaper and the following down_write();up_write(); cycle will > guarantee it is not operating on the mm before munlocking. > It does not make sense to call set_bit(MMF_UNSTABLE, &mm->flags) again after returning from __oom_reap_task_mm() because MMF_UNSTABLE is _aready_ set in the beginning of __oom_reap_task_mm(). void __oom_reap_task_mm(struct mm_struct *mm) { struct vm_area_struct *vma; /* * Tell all users of get_user/copy_from_user etc... that the content * is no longer stable. No barriers really needed because unmapping * should imply barriers already and the reader would hit a page fault * if it stumbled over a reaped memory. If MMF_UNSTABLE is already set, * reaping as already occurred so nothing left to do. */ if (test_and_set_bit(MMF_UNSTABLE, &mm->flags)) return; (...snipped...) } void exit_mmap(struct mm_struct *mm) { struct mmu_gather tlb; struct vm_area_struct *vma; unsigned long nr_accounted = 0; /* mm's last user has gone, and its about to be pulled down */ mmu_notifier_release(mm); if (unlikely(mm_is_oom_victim(mm))) { /* * Manually reap the mm to free as much memory as possible. * Nothing can be holding mm->mmap_sem here and the above call * to mmu_notifier_release(mm) ensures mmu notifier callbacks in * __oom_reap_task_mm() will not block. */ __oom_reap_task_mm(mm); /* * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. * This needs to be done before calling munlock_vma_pages_all(), * which clears VM_LOCKED, otherwise the oom reaper cannot * reliably test for it. If the oom reaper races with * munlock_vma_pages_all(), this can result in a kernel oops if * a pmd is zapped, for example, after follow_page_mask() has * checked pmd_none(). * * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will * guarantee that the oom reaper will not run on this mm again * after mmap_sem is dropped. */ set_bit(MMF_UNSTABLE, &mm->flags); down_write(&mm->mmap_sem); up_write(&mm->mmap_sem); } (...snipped...) } >> If the oom_lock serialization is removed, the OOM reaper will give up after (by default) >> 1 second even if current thread is immediately after set_bit(MMF_UNSTABLE, &mm->flags) from >> __oom_reap_task_mm() from exit_mmap(). Thus, this patch and the other patch which removes >> oom_lock serialization should be dropped. >> > > No, it shouldn't, lol. The oom reaper may give up because we have entered > __oom_reap_task_mm() by way of exit_mmap(), there's no other purpose for > it acting on the mm. This is very different from giving up by setting > MMF_OOM_SKIP, which it will wait for oom_free_timeout_ms to do unless the > thread can make forward progress here in exit_mmap(). Let's call "A" as a thread doing exit_mmap(), and "B" as the OOM reaper kernel thread. (1) "A" finds that unlikely(mm_is_oom_victim(mm)) == true. (2) "B" finds that test_bit(MMF_OOM_SKIP, &mm->flags) in oom_reap_task() is false. (3) "B" finds that !test_bit(MMF_UNSTABLE, &mm->flags) in oom_reap_task() is true. (4) "B" enters into oom_reap_task_mm(tsk, mm). (5) "B" finds that !down_read_trylock(&mm->mmap_sem) is false. (6) "B" finds that mm_has_blockable_invalidate_notifiers(mm) is false. (7) "B" finds that test_bit(MMF_UNSTABLE, &mm->flags) is false. (8) "B" enters into __oom_reap_task_mm(mm). (9) "A" finds that test_and_set_bit(MMF_UNSTABLE, &mm->flags) is false. (10) "A" is preempted by somebody else. (11) "B" finds that test_and_set_bit(MMF_UNSTABLE, &mm->flags) is true. (12) "B" leaves __oom_reap_task_mm(mm). (13) "B" leaves oom_reap_task_mm(). (14) "B" finds that time_after_eq(jiffies, mm->oom_free_expire) became true. (15) "B" finds that !test_bit(MMF_OOM_SKIP, &mm->flags) is true. (16) "B" calls set_bit(MMF_OOM_SKIP, &mm->flags). (17) "B" finds that test_bit(MMF_OOM_SKIP, &mm->flags) is true. (18) select_bad_process() finds that MMF_OOM_SKIP is already set. (19) out_of_memory() kills a new OOM victim. (20) "A" resumes execution and start reclaiming memory. because oom_lock serialization was already removed. > >>> down_write(&mm->mmap_sem); >>> up_write(&mm->mmap_sem); >>> } >> >>> @@ -637,25 +649,57 @@ static int oom_reaper(void *unused) >>> return 0; >>> } >>> >>> +/* >>> + * Millisecs to wait for an oom mm to free memory before selecting another >>> + * victim. >>> + */ >>> +static u64 oom_free_timeout_ms = 1000; >>> static void wake_oom_reaper(struct task_struct *tsk) >>> { >>> - /* tsk is already queued? */ >>> - if (tsk == oom_reaper_list || tsk->oom_reaper_list) >>> + /* >>> + * Set the reap timeout; if it's already set, the mm is enqueued and >>> + * this tsk can be ignored. >>> + */ >>> + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, >>> + jiffies + msecs_to_jiffies(oom_free_timeout_ms))) >>> return; >> >> "expire" must not be 0 in order to avoid double list_add(). See >> https://lore.kernel.org/lkml/201807130620.w6D6KiAJ093010@www262.sakura.ne.jp/T/#u . >> > > We should not allow oom_free_timeout_ms to be 0 for sure, I assume 1000 is > the sane minimum since we need to allow time for some memory freeing and > this will not be radically different from what existed before the patch > for the various backoffs. Or maybe you meant something else for "expire" > here? > I'm saying that jiffies + msecs_to_jiffies(oom_free_timeout_ms) == 0 will make tsk->signal->oom_mm->oom_free_expire == 0 and the list will be corrupted by allowing cmpxchg(&tsk->signal->oom_mm->oom_free_expire) to become true for twice.
David, Now that your patches are about to be dropped from linux-next.git , please try OOM lockup (CVE-2016-10723) mitigation patch ( https://marc.info/?l=linux-mm&m=153112243424285&w=4 ) and my cleanup patch ( [PATCH 1/2] at https://marc.info/?l=linux-mm&m=153119509215026&w=4 ) on top of linux.git . And please reply how was the result, for I'm currently asking Roman whether we can apply these patches before applying the cgroup-aware OOM killer.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -449,6 +449,13 @@ struct mm_struct { #ifdef CONFIG_MMU_NOTIFIER struct mmu_notifier_mm *mmu_notifier_mm; #endif +#ifdef CONFIG_MMU + /* + * When to give up on memory freeing from this mm after its + * threads have been oom killed, in jiffies. + */ + unsigned long oom_free_expire; +#endif #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS pgtable_t pmd_huge_pte; /* protected by page_table_lock */ #endif diff --git a/include/linux/sched.h b/include/linux/sched.h --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1163,7 +1163,8 @@ struct task_struct { #endif int pagefault_disabled; #ifdef CONFIG_MMU - struct task_struct *oom_reaper_list; + /* OOM victim queue for oom reaper, protected by oom_reaper_lock */ + struct list_head oom_reap_list; #endif #ifdef CONFIG_VMAP_STACK struct vm_struct *stack_vm_area; diff --git a/kernel/fork.c b/kernel/fork.c --- a/kernel/fork.c +++ b/kernel/fork.c @@ -842,6 +842,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) #ifdef CONFIG_FAULT_INJECTION tsk->fail_nth = 0; #endif +#ifdef CONFIG_MMU + INIT_LIST_HEAD(&tsk->oom_reap_list); +#endif return tsk; diff --git a/mm/mmap.c b/mm/mmap.c --- a/mm/mmap.c +++ b/mm/mmap.c @@ -3059,25 +3059,28 @@ void exit_mmap(struct mm_struct *mm) if (unlikely(mm_is_oom_victim(mm))) { /* * Manually reap the mm to free as much memory as possible. - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard - * this mm from further consideration. Taking mm->mmap_sem for - * write after setting MMF_OOM_SKIP will guarantee that the oom - * reaper will not run on this mm again after mmap_sem is - * dropped. - * * Nothing can be holding mm->mmap_sem here and the above call * to mmu_notifier_release(mm) ensures mmu notifier callbacks in * __oom_reap_task_mm() will not block. - * - * This needs to be done before calling munlock_vma_pages_all(), - * which clears VM_LOCKED, otherwise the oom reaper cannot - * reliably test it. */ mutex_lock(&oom_lock); __oom_reap_task_mm(mm); mutex_unlock(&oom_lock); - set_bit(MMF_OOM_SKIP, &mm->flags); + /* + * Now, set MMF_UNSTABLE to avoid racing with the oom reaper. + * This needs to be done before calling munlock_vma_pages_all(), + * which clears VM_LOCKED, otherwise the oom reaper cannot + * reliably test for it. If the oom reaper races with + * munlock_vma_pages_all(), this can result in a kernel oops if + * a pmd is zapped, for example, after follow_page_mask() has + * checked pmd_none(). + * + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will + * guarantee that the oom reaper will not run on this mm again + * after mmap_sem is dropped. + */ + set_bit(MMF_UNSTABLE, &mm->flags); down_write(&mm->mmap_sem); up_write(&mm->mmap_sem); } @@ -3105,6 +3108,7 @@ void exit_mmap(struct mm_struct *mm) unmap_vmas(&tlb, vma, 0, -1); free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); tlb_finish_mmu(&tlb, 0, -1); + set_bit(MMF_OOM_SKIP, &mm->flags); /* * Walk the list again, actually closing and freeing it, diff --git a/mm/oom_kill.c b/mm/oom_kill.c --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -41,6 +41,7 @@ #include <linux/kthread.h> #include <linux/init.h> #include <linux/mmu_notifier.h> +#include <linux/debugfs.h> #include <asm/tlb.h> #include "internal.h" @@ -476,7 +477,7 @@ bool process_shares_mm(struct task_struct *p, struct mm_struct *mm) */ static struct task_struct *oom_reaper_th; static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait); -static struct task_struct *oom_reaper_list; +static LIST_HEAD(oom_reaper_list); static DEFINE_SPINLOCK(oom_reaper_lock); void __oom_reap_task_mm(struct mm_struct *mm) @@ -519,10 +520,8 @@ void __oom_reap_task_mm(struct mm_struct *mm) } } -static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) +static void oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) { - bool ret = true; - /* * We have to make sure to not race with the victim exit path * and cause premature new oom victim selection: @@ -540,9 +539,8 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) mutex_lock(&oom_lock); if (!down_read_trylock(&mm->mmap_sem)) { - ret = false; trace_skip_task_reaping(tsk->pid); - goto unlock_oom; + goto out_oom; } /* @@ -551,69 +549,81 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm) * TODO: we really want to get rid of this ugly hack and make sure that * notifiers cannot block for unbounded amount of time */ - if (mm_has_blockable_invalidate_notifiers(mm)) { - up_read(&mm->mmap_sem); - schedule_timeout_idle(HZ); - goto unlock_oom; - } + if (mm_has_blockable_invalidate_notifiers(mm)) + goto out_mm; /* - * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't - * work on the mm anymore. The check for MMF_OOM_SKIP must run + * MMF_UNSTABLE is set by exit_mmap when the OOM reaper can't + * work on the mm anymore. The check for MMF_UNSTABLE must run * under mmap_sem for reading because it serializes against the * down_write();up_write() cycle in exit_mmap(). */ - if (test_bit(MMF_OOM_SKIP, &mm->flags)) { - up_read(&mm->mmap_sem); + if (test_bit(MMF_UNSTABLE, &mm->flags)) { trace_skip_task_reaping(tsk->pid); - goto unlock_oom; + goto out_mm; } trace_start_task_reaping(tsk->pid); - __oom_reap_task_mm(mm); + trace_finish_task_reaping(tsk->pid); pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n", task_pid_nr(tsk), tsk->comm, K(get_mm_counter(mm, MM_ANONPAGES)), K(get_mm_counter(mm, MM_FILEPAGES)), K(get_mm_counter(mm, MM_SHMEMPAGES))); +out_mm: up_read(&mm->mmap_sem); - - trace_finish_task_reaping(tsk->pid); -unlock_oom: +out_oom: mutex_unlock(&oom_lock); - return ret; } -#define MAX_OOM_REAP_RETRIES 10 static void oom_reap_task(struct task_struct *tsk) { - int attempts = 0; struct mm_struct *mm = tsk->signal->oom_mm; - /* Retry the down_read_trylock(mmap_sem) a few times */ - while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm)) - schedule_timeout_idle(HZ/10); + /* + * If this mm has either been fully unmapped, or the oom reaper has + * given up on it, nothing left to do except drop the refcount. + */ + if (test_bit(MMF_OOM_SKIP, &mm->flags)) + goto drop; - if (attempts <= MAX_OOM_REAP_RETRIES || - test_bit(MMF_OOM_SKIP, &mm->flags)) - goto done; + /* + * If this mm has already been reaped, doing so again will not likely + * free additional memory. + */ + if (!test_bit(MMF_UNSTABLE, &mm->flags)) + oom_reap_task_mm(tsk, mm); - pr_info("oom_reaper: unable to reap pid:%d (%s)\n", - task_pid_nr(tsk), tsk->comm); - debug_show_all_locks(); + if (time_after_eq(jiffies, mm->oom_free_expire)) { + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) { + pr_info("oom_reaper: unable to reap pid:%d (%s)\n", + task_pid_nr(tsk), tsk->comm); + debug_show_all_locks(); -done: - tsk->oom_reaper_list = NULL; + /* + * Reaping has failed for the timeout period, so give up + * and allow additional processes to be oom killed. + */ + set_bit(MMF_OOM_SKIP, &mm->flags); + } + goto drop; + } - /* - * Hide this mm from OOM killer because it has been either reaped or - * somebody can't call up_write(mmap_sem). - */ - set_bit(MMF_OOM_SKIP, &mm->flags); + if (test_bit(MMF_OOM_SKIP, &mm->flags)) + goto drop; + + /* Enqueue to be reaped again */ + spin_lock(&oom_reaper_lock); + list_add_tail(&tsk->oom_reap_list, &oom_reaper_list); + spin_unlock(&oom_reaper_lock); - /* Drop a reference taken by wake_oom_reaper */ + schedule_timeout_idle(HZ/10); + return; + +drop: + /* Drop the reference taken by wake_oom_reaper */ put_task_struct(tsk); } @@ -622,11 +632,13 @@ static int oom_reaper(void *unused) while (true) { struct task_struct *tsk = NULL; - wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL); + wait_event_freezable(oom_reaper_wait, + !list_empty(&oom_reaper_list)); spin_lock(&oom_reaper_lock); - if (oom_reaper_list != NULL) { - tsk = oom_reaper_list; - oom_reaper_list = tsk->oom_reaper_list; + if (!list_empty(&oom_reaper_list)) { + tsk = list_entry(oom_reaper_list.next, + struct task_struct, oom_reap_list); + list_del(&tsk->oom_reap_list); } spin_unlock(&oom_reaper_lock); @@ -637,25 +649,57 @@ static int oom_reaper(void *unused) return 0; } +/* + * Millisecs to wait for an oom mm to free memory before selecting another + * victim. + */ +static u64 oom_free_timeout_ms = 1000; static void wake_oom_reaper(struct task_struct *tsk) { - /* tsk is already queued? */ - if (tsk == oom_reaper_list || tsk->oom_reaper_list) + /* + * Set the reap timeout; if it's already set, the mm is enqueued and + * this tsk can be ignored. + */ + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, + jiffies + msecs_to_jiffies(oom_free_timeout_ms))) return; get_task_struct(tsk); spin_lock(&oom_reaper_lock); - tsk->oom_reaper_list = oom_reaper_list; - oom_reaper_list = tsk; + list_add(&tsk->oom_reap_list, &oom_reaper_list); spin_unlock(&oom_reaper_lock); trace_wake_reaper(tsk->pid); wake_up(&oom_reaper_wait); } +#ifdef CONFIG_DEBUG_FS +static int oom_free_timeout_ms_read(void *data, u64 *val) +{ + *val = oom_free_timeout_ms; + return 0; +} + +static int oom_free_timeout_ms_write(void *data, u64 val) +{ + if (val > 60 * 1000) + return -EINVAL; + + oom_free_timeout_ms = val; + return 0; +} +DEFINE_SIMPLE_ATTRIBUTE(oom_free_timeout_ms_fops, oom_free_timeout_ms_read, + oom_free_timeout_ms_write, "%llu\n"); +#endif /* CONFIG_DEBUG_FS */ + static int __init oom_init(void) { oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper"); +#ifdef CONFIG_DEBUG_FS + if (!IS_ERR(oom_reaper_th)) + debugfs_create_file("oom_free_timeout_ms", 0200, NULL, NULL, + &oom_free_timeout_ms_fops); +#endif return 0; } subsys_initcall(oom_init)
The oom reaper ensures forward progress by setting MMF_OOM_SKIP itself if it cannot reap an mm. This can happen for a variety of reasons, including: - the inability to grab mm->mmap_sem in a sufficient amount of time, - when the mm has blockable mmu notifiers that could cause the oom reaper to stall indefinitely, but we can also add a third when the oom reaper can "reap" an mm but doing so is unlikely to free any amount of memory: - when the mm's memory is mostly mlocked. When all memory is mlocked, the oom reaper will not be able to free any substantial amount of memory. It sets MMF_OOM_SKIP before the victim can unmap and free its memory in exit_mmap() and subsequent oom victims are chosen unnecessarily. This is trivial to reproduce if all eligible processes on the system have mlocked their memory: the oom killer calls panic() even though forward progress can be made. This is the same issue where the exit path sets MMF_OOM_SKIP before unmapping memory and additional processes can be chosen unnecessarily because the oom killer is racing with exit_mmap() and is separate from the oom reaper setting MMF_OOM_SKIP prematurely. We can't simply defer setting MMF_OOM_SKIP, however, because if there is a true oom livelock in progress, it never gets set and no additional killing is possible. To fix this, this patch introduces a per-mm reaping period, which is configurable through the new oom_free_timeout_ms file in debugfs and defaults to one second to match the current heuristics. This support requires that the oom reaper's list becomes a proper linked list so that other mm's may be reaped while waiting for an mm's timeout to expire. This replaces the current timeouts in the oom reaper: (1) when trying to grab mm->mmap_sem 10 times in a row with HZ/10 sleeps in between and (2) a HZ sleep if there are blockable mmu notifiers. It extends it with timeout to allow an oom victim to reach exit_mmap() before choosing additional processes unnecessarily. The exit path will now set MMF_OOM_SKIP only after all memory has been freed, so additional oom killing is justified, and rely on MMF_UNSTABLE to determine when it can race with the oom reaper. The oom reaper will now set MMF_OOM_SKIP only after the reap timeout has lapsed because it can no longer guarantee forward progress. Since the default oom_free_timeout_ms is one second, the same as current heuristics, there should be no functional change with this patch for users who do not tune it to be longer other than MMF_OOM_SKIP is set by exit_mmap() after free_pgtables(), which is the preferred behavior. The reaping timeout can intentionally be set for a substantial amount of time, such as 10s, since oom livelock is a very rare occurrence and it's better to optimize for preventing additional (unnecessary) oom killing than a scenario that is much more unlikely. Signed-off-by: David Rientjes <rientjes@google.com> --- v3: - oom_free_timeout_ms is now static per kbuild test robot v2: - configurable timeout period through debugfs - change mm->reap_timeout to mm->oom_free_expire and add more descriptive comment per akpm - add comment to describe task->oom_reap_list locking based on oom_reaper_lock per akpm - rework the exit_mmap() comment and split into two parts to be more descriptive about the locking and the issue with the oom reaper racing with munlock_vma_pages_all() per akpm --- include/linux/mm_types.h | 7 ++ include/linux/sched.h | 3 +- kernel/fork.c | 3 + mm/mmap.c | 26 +++++--- mm/oom_kill.c | 140 +++++++++++++++++++++++++-------------- 5 files changed, 119 insertions(+), 60 deletions(-)