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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 30 Jun 2023 14:19:51 -0700 Suren Baghdasaryan <surenb@google.com> 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?