| Message ID | 20230706011400.2949242-2-surenb@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Avoid memory corruption caused by per-VMA locks | expand |
* Suren Baghdasaryan <surenb@google.com> [230705 21:14]: > When forking a child process, parent write-protects an anonymous page > and COW-shares it with the child being forked using copy_present_pte(). > Parent's TLB is flushed right before we drop the parent's mmap_lock in > dup_mmap(). If we get a write-fault before that TLB flush in the parent, > and we end up replacing that anonymous page in the parent process in > do_wp_page() (because, COW-shared with the child), this might lead to > some stale writable TLB entries targeting the wrong (old) page. > Similar issue happened in the past with userfaultfd (see flush_tlb_page() > call inside do_wp_page()). > Lock VMAs of the parent process when forking a child, which prevents > concurrent page faults during fork operation and avoids this issue. > This fix can potentially regress some fork-heavy workloads. Kernel build > time did not show noticeable regression on a 56-core machine while a > stress test mapping 10000 VMAs and forking 5000 times in a tight loop > shows ~7% regression. If such fork time regression is unacceptable, > disabling CONFIG_PER_VMA_LOCK should restore its performance. Further > optimizations are possible if this regression proves to be problematic. > > Suggested-by: David Hildenbrand <david@redhat.com> > Reported-by: Jiri Slaby <jirislaby@kernel.org> > Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/ > Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com> > Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/ > Reported-by: Jacob Young <jacobly.alt@gmail.com> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624 > Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") > Cc: stable@vger.kernel.org > Signed-off-by: Suren Baghdasaryan <surenb@google.com> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com> > --- > kernel/fork.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/kernel/fork.c b/kernel/fork.c > index b85814e614a5..2ba918f83bde 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, > retval = -EINTR; > goto fail_uprobe_end; > } > +#ifdef CONFIG_PER_VMA_LOCK > + /* Disallow any page faults before calling flush_cache_dup_mm */ > + for_each_vma(old_vmi, mpnt) > + vma_start_write(mpnt); > + vma_iter_set(&old_vmi, 0); > +#endif > flush_cache_dup_mm(oldmm); > uprobe_dup_mmap(oldmm, mm); > /* > -- > 2.41.0.255.g8b1d071c50-goog >
On 06.07.23 03:13, Suren Baghdasaryan wrote: > When forking a child process, parent write-protects an anonymous page > and COW-shares it with the child being forked using copy_present_pte(). > Parent's TLB is flushed right before we drop the parent's mmap_lock in > dup_mmap(). If we get a write-fault before that TLB flush in the parent, > and we end up replacing that anonymous page in the parent process in > do_wp_page() (because, COW-shared with the child), this might lead to > some stale writable TLB entries targeting the wrong (old) page. > Similar issue happened in the past with userfaultfd (see flush_tlb_page() > call inside do_wp_page()). > Lock VMAs of the parent process when forking a child, which prevents > concurrent page faults during fork operation and avoids this issue. > This fix can potentially regress some fork-heavy workloads. Kernel build > time did not show noticeable regression on a 56-core machine while a > stress test mapping 10000 VMAs and forking 5000 times in a tight loop > shows ~7% regression. If such fork time regression is unacceptable, > disabling CONFIG_PER_VMA_LOCK should restore its performance. Further > optimizations are possible if this regression proves to be problematic. > > Suggested-by: David Hildenbrand <david@redhat.com> > Reported-by: Jiri Slaby <jirislaby@kernel.org> > Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/ > Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com> > Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/ > Reported-by: Jacob Young <jacobly.alt@gmail.com> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624 > Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") > Cc: stable@vger.kernel.org > Signed-off-by: Suren Baghdasaryan <surenb@google.com> Acked-by: David Hildenbrand <david@redhat.com> Feel free to keep my ACK on minor changes.
diff --git a/kernel/fork.c b/kernel/fork.c index b85814e614a5..2ba918f83bde 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, retval = -EINTR; goto fail_uprobe_end; } +#ifdef CONFIG_PER_VMA_LOCK + /* Disallow any page faults before calling flush_cache_dup_mm */ + for_each_vma(old_vmi, mpnt) + vma_start_write(mpnt); + vma_iter_set(&old_vmi, 0); +#endif flush_cache_dup_mm(oldmm); uprobe_dup_mmap(oldmm, mm); /*
When forking a child process, parent write-protects an anonymous page and COW-shares it with the child being forked using copy_present_pte(). Parent's TLB is flushed right before we drop the parent's mmap_lock in dup_mmap(). If we get a write-fault before that TLB flush in the parent, and we end up replacing that anonymous page in the parent process in do_wp_page() (because, COW-shared with the child), this might lead to some stale writable TLB entries targeting the wrong (old) page. Similar issue happened in the past with userfaultfd (see flush_tlb_page() call inside do_wp_page()). Lock VMAs of the parent process when forking a child, which prevents concurrent page faults during fork operation and avoids this issue. This fix can potentially regress some fork-heavy workloads. Kernel build time did not show noticeable regression on a 56-core machine while a stress test mapping 10000 VMAs and forking 5000 times in a tight loop shows ~7% regression. If such fork time regression is unacceptable, disabling CONFIG_PER_VMA_LOCK should restore its performance. Further optimizations are possible if this regression proves to be problematic. Suggested-by: David Hildenbrand <david@redhat.com> Reported-by: Jiri Slaby <jirislaby@kernel.org> Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/ Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com> Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/ Reported-by: Jacob Young <jacobly.alt@gmail.com> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624 Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") Cc: stable@vger.kernel.org Signed-off-by: Suren Baghdasaryan <surenb@google.com> --- kernel/fork.c | 6 ++++++ 1 file changed, 6 insertions(+)