[PATCH v2 3/3] fork: lock VMAs of the parent process when forking

Suren Baghdasaryan surenb at google.com
Sun Jul 9 05:22:13 AEST 2023

On Sat, Jul 8, 2023 at 12:12 PM Suren Baghdasaryan <surenb at google.com> 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:

> Suggested-by: David Hildenbrand <david at redhat.com>
> Reported-by: Jiri Slaby <jirislaby at kernel.org>
> Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/
> Reported-by: Holger Hoffstätte <holger at applied-asynchrony.com>
> Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/
> Reported-by: Jacob Young <jacobly.alt at 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 at vger.kernel.org
> Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> ---
>  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;
> --

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