Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults

[Suren Baghdasaryan]

On Sun, Jul 2, 2023 at 5:50 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 30 Jun 2023 14:19:51 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> > When per-VMA locks were introduced in [1] several types of page faults
> > would still fall back to mmap_lock to keep the patchset simple. Among them
> > are swap and userfault pages. The main reason for skipping those cases was
> > the fact that mmap_lock could be dropped while handling these faults and
> > that required additional logic to be implemented.
> > Implement the mechanism to allow per-VMA locks to be dropped for these
> > cases.
> > First, change handle_mm_fault to drop per-VMA locks when returning
> > VM_FAULT_RETRY or VM_FAULT_COMPLETED to be consistent with the way
> > mmap_lock is handled. Then change folio_lock_or_retry to accept vm_fault
> > and return vm_fault_t which simplifies later patches. Finally allow swap
> > and uffd page faults to be handled under per-VMA locks by dropping per-VMA
> > and retrying, the same way it's done under mmap_lock.
> > Naturally, once VMA lock is dropped that VMA should be assumed unstable
> > and can't be used.
>
> Is there any measurable performance benefit from this?

Good point. I haven't measured it but assume it will have the same
effect as for other page fault cases handled under per-VMA locks
(mmap_lock contention reduction). I'll try to create a test to measure
the effects.
Thanks,
Suren.