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

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Message-ID: <CAJuCfpG6BBP+fjV9oyBx3SNiKhiafPzM9vV9bx_goO2aZzAptg@mail.gmail.com>
Date:   Fri, 4 Aug 2023 17:34:06 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mateusz Guzik <mjguzik@...il.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 Fri, Aug 4, 2023 at 5:26 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Fri, Aug 4, 2023 at 5:15 PM 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.
>
> That check will prevent re-locking but if vma is not already locked
> then the call will proceed with obtaining the lock and setting
> vma->vm_lock_seq to mm->mm_lock_seq.

The optimization Mateusz describes looks valid to me. If there is
nobody else to fault a page and mm_users is stable (which I think it
is because we are holding mmap_lock for write) then we can skip vma
locking, I think.

>
> >
> > 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.
> >
> >           Linus