On Wed, Jul 5, 2023 at 5:33 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230705 20:20]: > > On Wed, Jul 5, 2023 at 4:07 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [230705 13:24]: > > > > On Wed, Jul 5, 2023 at 10:14 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > > > > > > > > > On 05.07.23 19:12, 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 ~5% 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@xxxxxxxxxx> > > > > > > Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx> > > > > > > Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@xxxxxxxxxx/ > > > > > > Reported-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx> > > > > > > Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@xxxxxxxxxxxxxxxxxxxxxx/ > > > > > > Reported-by: Jacob Young <jacobly.alt@xxxxxxxxx> > > > > > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624 > > > > > > Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") > > > > > > Cc: stable@xxxxxxxxxxxxxxx > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > > > > > --- > > > > > > kernel/fork.c | 6 ++++++ > > > > > > 1 file changed, 6 insertions(+) > > > > > > > > > > > > diff --git a/kernel/fork.c b/kernel/fork.c > > > > > > index b85814e614a5..403bc2b72301 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_init(&old_vmi, oldmm, 0); > > > > > > vma_iter_set(&old_vmi, 0) is probably what you want here. > > > > Ok, I send another version with that. > > > > > > > > > > > +#endif > > > > > > flush_cache_dup_mm(oldmm); > > > > > > uprobe_dup_mmap(oldmm, mm); > > > > > > /* > > > > > > > > > > The old version was most probably fine as well, but this certainly looks > > > > > even safer. > > > > > > > > > > Acked-by: David Hildenbrand <david@xxxxxxxxxx> > > > > > > I think this is overkill and believe setting the vma_start_write() will > > > synchronize with any readers since it's using the per-vma rw semaphore > > > in write mode. Anything faulting will need to finish before the fork > > > continues and faults during the fork will fall back to a read lock of > > > the mmap_lock. Is there a possibility of populate happening outside the > > > mmap_write lock/vma_lock? > > > > Yes, I think we understand the loss of concurrency in the parent's > > ability to fault pages while forking. Is that a real problem though? > > No, I don't think that part is an issue at all. I wanted to be sure I > didn't miss something. > > > > > > > > > Was your benchmarking done with this loop at the start? > > > > No, it was done with the initial version where the lock was inside the > > existing loop. I just reran the benchmark and while kernel compilation > > times did not change, the stress test shows ~7% regression now, > > probably due to that additional tree walk. I'll update that number in > > the new patch. > > ..but I expected a performance hit and didn't understand why you updated > the patch this way. It would probably only happen on really big trees > though and, ah, the largest trees I see are from the android side. I'd > wager the impact will be felt more when larger trees encounter smaller > CPU cache. My test has 10000 vmas and even for Android that's a stretch (the highest number I've seen was ~4000). We can think of a less restrictive solution if this proves to be a problem for some workloads but for now I would prefer to fix this in a safe way and possibly improve that later. The alternative is to revert this completely and we get no more testing until the next release. > > Thanks, > Liam
