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

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Date:   Mon, 3 Jul 2023 15:27:09 +0000
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     willy@...radead.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, michel@...pinasse.org,
        liam.howlett@...cle.com, jglisse@...gle.com, vbabka@...e.cz,
        minchan@...gle.com, dave@...olabs.net, punit.agrawal@...edance.com,
        lstoakes@...il.com, hdanton@...a.com, apopple@...dia.com,
        peterx@...hat.com, ying.huang@...el.com, david@...hat.com,
        yuzhao@...gle.com, dhowells@...hat.com, hughd@...gle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults

On Sun, Jul 2, 2023 at 5:50 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Fri, 30 Jun 2023 14:19:51 -0700 Suren Baghdasaryan <surenb@...gle.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?

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.