On Sat, Jul 8, 2023 at 12:12 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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. Sending this earlier version of the patch per request from Linus and with his explanation here: https://lore.kernel.org/all/CAHk-=wi-99-DyMOGywTbjRnRRC+XfpPm=r=pei4A=MEL0QDBXA@xxxxxxxxxxxxxx/ > > 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 | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kernel/fork.c b/kernel/fork.c > index b85814e614a5..d2e12b6d2b18 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -686,6 +686,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, > for_each_vma(old_vmi, mpnt) { > struct file *file; > > + vma_start_write(mpnt); > if (mpnt->vm_flags & VM_DONTCOPY) { > vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt)); > continue; > -- > 2.41.0.390.g38632f3daf-goog >
