Re: [PATCH v4 1/2] fork: lock VMAs of the parent process when forking

[David Hildenbrand]

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@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>

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Feel free to keep my ACK on minor changes.

--
Cheers,

David / dhildenb