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

[Mateusz Guzik]

On 8/5/23, Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> On Fri, Aug 4, 2023 at 6:06 PM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>>
>> On 8/5/23, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik@xxxxxxxxx> 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@xxxxxxxxxxxxxx/
>> >
>> > 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,
-- 
Mateusz Guzik <mjguzik gmail.com>