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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Sat, 8 Jul 2023 12:22:13 -0700
Message-ID: <CAJuCfpFBnweDUBnxEZQbqxYNtrY08C+1LxYsB_5zaMhrb=AHEg@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking
To:     torvalds@linux-foundation.org
Cc:     akpm@linux-foundation.org, regressions@leemhuis.info,
        bagasdotme@gmail.com, jacobly.alt@gmail.com, willy@infradead.org,
        liam.howlett@oracle.com, david@redhat.com, peterx@redhat.com,
        ldufour@linux.ibm.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        linux-arm-kernel@lists.infradead.org, gregkh@linuxfoundation.org,
        regressions@lists.linux.dev, Jiri Slaby <jirislaby@kernel.org>,
        =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>,
        stable@vger.kernel.org
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On Sat, Jul 8, 2023 at 12:12=E2=80=AFPM Suren Baghdasaryan <surenb@google.c=
om> 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-=3Dwi-99-DyMOGywTbjRnRRC+XfpPm=3Dr=3Dpei4A=
=3DMEL0QDBXA@mail.gmail.com/

>
> 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 Hoffst=C3=A4tte <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=3D217624
> Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling firs=
t")
> Cc: stable@vger.kernel.org
> Signed-off-by: Suren Baghdasaryan <surenb@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(mp=
nt));
>                         continue;
> --
> 2.41.0.390.g38632f3daf-goog
>