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

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Message-Id: <20230702105038.5d0f729109d329013af4caa3@linux-foundation.org>
Date:   Sun, 2 Jul 2023 10:50:38 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     willy@...radead.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
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        viro@...iv.linux.org.uk, brauner@...nel.org,
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        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 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?