Re: [PATCH v9 00/17] reimplement per-vma lock as a refcount

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Jan 13, 2025 at 5:49 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 13 Jan 2025 12:14:19 +0000 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
>
> > A nit on subject, I mean this is part of what this series does, and hey -
> > we have only so much text to put in here - but isn't this both
> > reimplementing per-VMA lock as a refcount _and_ importantly allocating VMAs
> > using the RCU typesafe mechanism?
> >
> > Do we have to do both in one series? Can we split this out? I mean maybe
> > that's just churny and unnecessary, but to me this series is 'allocate VMAs
> > RCU safe and refcount VMA lock' or something like this. Maybe this is
> > nitty... but still :)
> >
> > One general comment here - this is a really major change in how this stuff
> > works, and yet I don't see any tests anywhere in the series.
> >
> > I know it's tricky to write tests for this, but the new VMA testing
> > environment should make it possible to test a _lot_ more than we previously
> > could.
> >
> > However due to some (*ahem*) interesting distribution of where functions
> > are, most notably stuff in kernel/fork.c, I guess we can't test
> > _everything_ there effectively.
> >
> > But I do feel like we should be able to do better than having absolutely no
> > testing added for this?
> >
> > I think there's definitely quite a bit you could test now, at least in
> > asserting fundamentals in tools/testing/vma/vma.c.
> >
> > This can cover at least detached state asserts in various scenarios.
> >
> > But that won't cover off the really gnarly stuff here around RCU slab
> > allocation, and determining precisely how to test that in a sensible way is
> > maybe less clear.
> >
> > But I'd like to see _something_ here please, this is more or less
> > fundamentally changing how all VMAs are allocated and to just have nothing
> > feels unfortunate.
> >
> > I'm already nervous because we've hit issues coming up to v9 and we're not
> > 100% sure if a recent syzkaller is related to these changes or not, I'm not
> > sure how much we can get assurances with tests but I'd like something.
>
> Thanks.
>
> Yes, we're at -rc7 and this series is rather in panic mode and it seems
> unnecessarily risky so I'm inclined to set it aside for this cycle.
>
> If the series is considered super desirable and if people are confident
> that we can address any remaining glitches during two months of -rc
> then sure, we could push the envelope a bit.  But I don't believe this
> is the case so I'm thinking let's give ourselves another cycle to get
> this all sorted out?

I didn't think this series was in panic mode with one real issue that
is not hard to address (memory ordering in
__refcount_inc_not_zero_limited()) but I'm obviously biased and might
be missing the big picture. In any case, if it makes people nervous I
have no objections to your plan.
Thanks,
Suren.