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

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Message-ID: <CAGudoHFsxmiezZREyVkrPYBBij3u9SNaxLWipOOPy5S+5pJcZA@mail.gmail.com>
Date:   Sat, 5 Aug 2023 03:06:13 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        regressions@...mhuis.info, bagasdotme@...il.com,
        jacobly.alt@...il.com, willy@...radead.org,
        liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
        ldufour@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        regressions@...ts.linux.dev, Jiri Slaby <jirislaby@...nel.org>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking

On 8/5/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik@...il.com> wrote:
>>
>> I know of these guys, I think they are excluded as is -- they go
>> through access_remote_vm, starting with:
>>         if (mmap_read_lock_killable(mm))
>>                 return 0;
>>
>> while dup_mmap already write locks the parent's mm.
>
> Oh, you're only worried about vma_start_write()?
>
> That's a non-issue. It doesn't take the lock normally, since it starts off
> with
>
>         if (__is_vma_write_locked(vma, &mm_lock_seq))
>                 return;
>
> which catches on the lock sequence number already being set.
>
> So no extra locking there.
>
> Well, technically there's extra locking because the code stupidly
> doesn't initialize new vma allocations to the right sequence number,
> but that was talked about here:
>
>
> https://lore.kernel.org/all/CAHk-=wiCrWAoEesBuoGoqqufvesicbGp3cX0LyKgEvsFaZNpDA@mail.gmail.com/
>
> and it's a separate issue.
>

I'm going to bet one beer this is the issue.

The patch I'm responding to only consists of adding the call to
vma_start_write and claims the 5% slowdown from it, while fixing
crashes if the forking process is multithreaded.

For the fix to work it has to lock something against the parent.

        VMA_ITERATOR(old_vmi, oldmm, 0);
[..]
        for_each_vma(old_vmi, mpnt) {
[..]
                vma_start_write(mpnt);

the added line locks an obj in the parent's vm space.

The problem you linked looks like pessimization for freshly allocated
vmas, but that's what is being operated on here.

-- 
Mateusz Guzik <mjguzik gmail.com>