Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Sep 1, 2022 at 1:58 PM Kent Overstreet
<kent.overstreet@linux.dev> wrote:
>
> On Thu, Sep 01, 2022 at 10:34:48AM -0700, Suren Baghdasaryan wrote:
> > Resending to fix the issue with the In-Reply-To tag in the original
> > submission at [4].
> >
> > This is a proof of concept for per-vma locks idea that was discussed
> > during SPF [1] discussion at LSF/MM this year [2], which concluded with
> > suggestion that “a reader/writer semaphore could be put into the VMA
> > itself; that would have the effect of using the VMA as a sort of range
> > lock. There would still be contention at the VMA level, but it would be an
> > improvement.” This patchset implements this suggested approach.
> >
> > When handling page faults we lookup the VMA that contains the faulting
> > page under RCU protection and try to acquire its lock. If that fails we
> > fall back to using mmap_lock, similar to how SPF handled this situation.
> >
> > One notable way the implementation deviates from the proposal is the way
> > VMAs are marked as locked. Because during some of mm updates multiple
> > VMAs need to be locked until the end of the update (e.g. vma_merge,
> > split_vma, etc). Tracking all the locked VMAs, avoiding recursive locks
> > and other complications would make the code more complex. Therefore we
> > provide a way to "mark" VMAs as locked and then unmark all locked VMAs
> > all at once. This is done using two sequence numbers - one in the
> > vm_area_struct and one in the mm_struct. VMA is considered locked when
> > these sequence numbers are equal. To mark a VMA as locked we set the
> > sequence number in vm_area_struct to be equal to the sequence number
> > in mm_struct. To unlock all VMAs we increment mm_struct's seq number.
> > This allows for an efficient way to track locked VMAs and to drop the
> > locks on all VMAs at the end of the update.
>
> I like it - the sequence numbers are a stroke of genuius. For what it's doing
> the patchset seems almost small.

Thanks for reviewing it!

>
> Two complaints so far:
>  - I don't like the vma_mark_locked() name. To me it says that the caller
>    already took or is taking the lock and this function is just marking that
>    we're holding the lock, but it's really taking a different type of lock. But
>    this function can block, it really is taking a lock, so it should say that.
>
>    This is AFAIK a new concept, not sure I'm going to have anything good either,
>    but perhaps vma_lock_multiple()?

I'm open to name suggestions but vma_lock_multiple() is a bit
confusing to me. Will wait for more suggestions.

>
>  - I don't like the #ifdef and the separate fallback path in the fault handlers.
>
>    Can we make find_and_lock_anon_vma() do the right thing, and not fail unless
>    e.g. there isn't a vma at that address? Just have it wait for vm_lock_seq to
>    change and then retry if needed.

I think it can be done but would come with additional complexity. I
was really trying to keep things as simple as possible after SPF got
shot down on the grounds of complexity. I hope to start simple and
improve only when necessary.

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