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From:   Mateusz Guzik <mjguzik@gmail.com>
Date:   Wed, 9 Aug 2023 23:07:45 +0200
Message-ID: <CAGudoHEU62Rpp+uJakCFgKC36y3th=Qx+jE+b7KJh6_jeH7cNg@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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 8/5/23, Suren Baghdasaryan <surenb@google.com> wrote:
> On Fri, Aug 4, 2023 at 6:06=E2=80=AFPM Mateusz Guzik <mjguzik@gmail.com> =
wrote:
>>
>> On 8/5/23, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>> > On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik@gmail.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-=3DwiCrWAoEesBuoGoqqufvesicbGp3cX0LyK=
gEvsFaZNpDA@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.
>
> Sorry, now I'm having trouble understanding the problem you are
> describing. We are locking the parent's vma before copying it and the
> newly created vma is locked before it's added into the vma tree. What
> is the problem then?
>

Sorry for the late reply!

Looks there has been a bunch of weird talking past one another in this
thread and I don't think trying to straighten it all out is worth any
time.

I think at least the two of us agree that if a single-threaded process
enters dup_mmap an
down_writes the mmap semaphore, then no new thread can pop up in said
process, thus no surprise page faults from that angle. 3rd parties are
supposed to interfaces like access_remote_vm, which down_read said
semaphore and are consequently also not a problem. The only worry here
is that someone is messing with another process memory without the
semaphore, but is very unlikely and patchable in the worst case -- but
someone(tm) has to audit. With all these conditions satisfied one can
elide vma_start_write for a perf win.

Finally, I think we agreed you are going to do the audit ;)

Cheers,
--=20
Mateusz Guzik <mjguzik gmail.com>