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

[Suren Baghdasaryan <surenb@google.com>]

On Sun, Sep 11, 2022 at 2:35 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 9/2/22 01:26, Suren Baghdasaryan wrote:
> > 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.
>
> Well, it does act like a vma_write_lock(), no? So why not that name. The
> checking function for it is even called vma_assert_write_locked().
>
> We just don't provide a single vma_write_unlock(), but a
> vma_mark_unlocked_all(), that could be instead named e.g.
> vma_write_unlock_all().
> But it's called on a mm, so maybe e.g. mm_vma_write_unlock_all()?

Thank you for your suggestions, Vlastimil! vma_write_lock() sounds
good to me. For vma_mark_unlocked_all() replacement, I would prefer
vma_write_unlock_all() which keeps the vma_write_XXX naming pattern to
indicate that these are operating on the same locks. If the fact that
it accepts mm_struct as a parameter is an issue then maybe
vma_write_unlock_mm() ?

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