| Message ID | 20230818081727.4181963-1-tongtiangen@huawei.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC,v2-next] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs() | expand |
On 2023/8/18 16:17, Tong Tiangen wrote: > We found a softlock issue in our test, analyzed the logs, and found that > the relevant CPU call trace as follows: > > CPU0: > _do_fork > -> copy_process() > -> write_lock_irq(&tasklist_lock) //Disable irq,waiting for > //tasklist_lock > > CPU1: > wp_page_copy() > ->pte_offset_map_lock() > -> spin_lock(&page->ptl); //Hold page->ptl > -> ptep_clear_flush() > -> flush_tlb_others() ... > -> smp_call_function_many() > -> arch_send_call_function_ipi_mask() > -> csd_lock_wait() //Waiting for other CPUs respond > //IPI > > CPU2: > collect_procs_anon() > -> read_lock(&tasklist_lock) //Hold tasklist_lock > ->for_each_process(tsk) > -> page_mapped_in_vma() > -> page_vma_mapped_walk() > -> map_pte() > ->spin_lock(&page->ptl) //Waiting for page->ptl > > We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2 > unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result, > softlockup is triggered. > > For collect_procs_anon(), we will not modify the tasklist, but only perform > read traversal. Therefore, we can use rcu lock instead of spin lock > tasklist_lock, from this, we can break the softlock chain above. > > The same logic can also be applied to: > - collect_procs_file() > - collect_procs_fsdax() > - collect_procs_ksm() > - find_early_kill_thread() > > Signed-off-by: Tong Tiangen <tongtiangen@huawei.com> > --- > v2: > - 1. Modify the title description. > - 2. Optimize the implementation of find_early_kill_thread() without > functional changes. > --- Those changes are fine to me， please fix the comment mentioned by Noaoya. > mm/ksm.c | 4 ++-- > mm/memory-failure.c | 33 +++++++++++++++++++-------------- > 2 files changed, 21 insertions(+), 16 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 6b7b8928fb96..dcbc0c7f68e7 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2919,7 +2919,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill, > struct anon_vma *av = rmap_item->anon_vma; > > anon_vma_lock_read(av); > - read_lock(&tasklist_lock); > + rcu_read_lock(); > for_each_process(tsk) { > struct anon_vma_chain *vmac; > unsigned long addr; > @@ -2938,7 +2938,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill, > } > } > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > anon_vma_unlock_read(av); > } > } > diff --git a/mm/memory-failure.c b/mm/memory-failure.c > index 7b01fffe7a79..4f3081f47798 100644 > --- a/mm/memory-failure.c > +++ b/mm/memory-failure.c > @@ -546,24 +546,29 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail, > * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO) > * on behalf of the thread group. Return task_struct of the (first found) > * dedicated thread if found, and return NULL otherwise. > - * > - * We already hold read_lock(&tasklist_lock) in the caller, so we don't > - * have to call rcu_read_lock/unlock() in this function. > */ > static struct task_struct *find_early_kill_thread(struct task_struct *tsk) > { > struct task_struct *t; > + bool found = false; > > + rcu_read_lock(); > for_each_thread(tsk, t) { > if (t->flags & PF_MCE_PROCESS) { > - if (t->flags & PF_MCE_EARLY) > - return t; > + if (t->flags & PF_MCE_EARLY) { > + found = true; > + break; > + } > } else { > - if (sysctl_memory_failure_early_kill) > - return t; > + if (sysctl_memory_failure_early_kill) { > + found = true; > + break; > + } > } > } > - return NULL; > + rcu_read_unlock(); > + > + return found ? t : NULL; > } > > /* > @@ -609,7 +614,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill, > return; > > pgoff = page_to_pgoff(page); > - read_lock(&tasklist_lock); > + rcu_read_lock(); > for_each_process(tsk) { > struct anon_vma_chain *vmac; > struct task_struct *t = task_early_kill(tsk, force_early); > @@ -626,7 +631,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill, > add_to_kill_anon_file(t, page, vma, to_kill); > } > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > anon_vma_unlock_read(av); > } > > @@ -642,7 +647,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill, > pgoff_t pgoff; > > i_mmap_lock_read(mapping); > - read_lock(&tasklist_lock); > + rcu_read_lock(); > pgoff = page_to_pgoff(page); > for_each_process(tsk) { > struct task_struct *t = task_early_kill(tsk, force_early); > @@ -662,7 +667,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill, > add_to_kill_anon_file(t, page, vma, to_kill); > } > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > i_mmap_unlock_read(mapping); > } > > @@ -685,7 +690,7 @@ static void collect_procs_fsdax(struct page *page, > struct task_struct *tsk; > > i_mmap_lock_read(mapping); > - read_lock(&tasklist_lock); > + rcu_read_lock(); > for_each_process(tsk) { > struct task_struct *t = task_early_kill(tsk, true); > > @@ -696,7 +701,7 @@ static void collect_procs_fsdax(struct page *page, > add_to_kill_fsdax(t, page, vma, to_kill, pgoff); > } > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > i_mmap_unlock_read(mapping); > } > #endif /* CONFIG_FS_DAX */
diff --git a/mm/ksm.c b/mm/ksm.c index 6b7b8928fb96..dcbc0c7f68e7 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2919,7 +2919,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill, struct anon_vma *av = rmap_item->anon_vma; anon_vma_lock_read(av); - read_lock(&tasklist_lock); + rcu_read_lock(); for_each_process(tsk) { struct anon_vma_chain *vmac; unsigned long addr; @@ -2938,7 +2938,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill, } } } - read_unlock(&tasklist_lock); + rcu_read_unlock(); anon_vma_unlock_read(av); } } diff --git a/mm/memory-failure.c b/mm/memory-failure.c index 7b01fffe7a79..4f3081f47798 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -546,24 +546,29 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail, * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO) * on behalf of the thread group. Return task_struct of the (first found) * dedicated thread if found, and return NULL otherwise. - * - * We already hold read_lock(&tasklist_lock) in the caller, so we don't - * have to call rcu_read_lock/unlock() in this function. */ static struct task_struct *find_early_kill_thread(struct task_struct *tsk) { struct task_struct *t; + bool found = false; + rcu_read_lock(); for_each_thread(tsk, t) { if (t->flags & PF_MCE_PROCESS) { - if (t->flags & PF_MCE_EARLY) - return t; + if (t->flags & PF_MCE_EARLY) { + found = true; + break; + } } else { - if (sysctl_memory_failure_early_kill) - return t; + if (sysctl_memory_failure_early_kill) { + found = true; + break; + } } } - return NULL; + rcu_read_unlock(); + + return found ? t : NULL; } /* @@ -609,7 +614,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill, return; pgoff = page_to_pgoff(page); - read_lock(&tasklist_lock); + rcu_read_lock(); for_each_process(tsk) { struct anon_vma_chain *vmac; struct task_struct *t = task_early_kill(tsk, force_early); @@ -626,7 +631,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill, add_to_kill_anon_file(t, page, vma, to_kill); } } - read_unlock(&tasklist_lock); + rcu_read_unlock(); anon_vma_unlock_read(av); } @@ -642,7 +647,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill, pgoff_t pgoff; i_mmap_lock_read(mapping); - read_lock(&tasklist_lock); + rcu_read_lock(); pgoff = page_to_pgoff(page); for_each_process(tsk) { struct task_struct *t = task_early_kill(tsk, force_early); @@ -662,7 +667,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill, add_to_kill_anon_file(t, page, vma, to_kill); } } - read_unlock(&tasklist_lock); + rcu_read_unlock(); i_mmap_unlock_read(mapping); } @@ -685,7 +690,7 @@ static void collect_procs_fsdax(struct page *page, struct task_struct *tsk; i_mmap_lock_read(mapping); - read_lock(&tasklist_lock); + rcu_read_lock(); for_each_process(tsk) { struct task_struct *t = task_early_kill(tsk, true); @@ -696,7 +701,7 @@ static void collect_procs_fsdax(struct page *page, add_to_kill_fsdax(t, page, vma, to_kill, pgoff); } } - read_unlock(&tasklist_lock); + rcu_read_unlock(); i_mmap_unlock_read(mapping); } #endif /* CONFIG_FS_DAX */
We found a softlock issue in our test, analyzed the logs, and found that the relevant CPU call trace as follows: CPU0: _do_fork -> copy_process() -> write_lock_irq(&tasklist_lock) //Disable irq,waiting for //tasklist_lock CPU1: wp_page_copy() ->pte_offset_map_lock() -> spin_lock(&page->ptl); //Hold page->ptl -> ptep_clear_flush() -> flush_tlb_others() ... -> smp_call_function_many() -> arch_send_call_function_ipi_mask() -> csd_lock_wait() //Waiting for other CPUs respond //IPI CPU2: collect_procs_anon() -> read_lock(&tasklist_lock) //Hold tasklist_lock ->for_each_process(tsk) -> page_mapped_in_vma() -> page_vma_mapped_walk() -> map_pte() ->spin_lock(&page->ptl) //Waiting for page->ptl We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2 unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result, softlockup is triggered. For collect_procs_anon(), we will not modify the tasklist, but only perform read traversal. Therefore, we can use rcu lock instead of spin lock tasklist_lock, from this, we can break the softlock chain above. The same logic can also be applied to: - collect_procs_file() - collect_procs_fsdax() - collect_procs_ksm() - find_early_kill_thread() Signed-off-by: Tong Tiangen <tongtiangen@huawei.com> --- v2: - 1. Modify the title description. - 2. Optimize the implementation of find_early_kill_thread() without functional changes. --- mm/ksm.c | 4 ++-- mm/memory-failure.c | 33 +++++++++++++++++++-------------- 2 files changed, 21 insertions(+), 16 deletions(-)