Re: [PATCH 0/6] Handle more faults under the VMA lock

From: Suren Baghdasaryan <surenb@google.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org
Subject: Re: [PATCH 0/6] Handle more faults under the VMA lock
Date: Wed, 27 Sep 2023 15:18:38 -0700	[thread overview]
Message-ID: <CAJuCfpE69Cbt7AHL2iQuTU578hP5wi0soOVp6i67QE1SeCrAHQ@mail.gmail.com> (raw)
In-Reply-To: <20230927052505.2855872-1-willy@infradead.org>

On Tue, Sep 26, 2023 at 10:25 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> At this point, we're handling the majority of file-backed page faults
> under the VMA lock, using the ->map_pages entry point.  This patch set
> attempts to expand that for the following siutations:
>
>  - We have to do a read.  This could be because we've hit the point in
>    the readahead window where we need to kick off the next readahead,
>    or because the page is simply not present in cache.
>  - We're handling a write fault.  Most applications don't do I/O by writes
>    to shared mmaps for very good reasons, but some do, and it'd be nice
>    to not make that slow unnecessarily.
>  - We're doing a COW of a private mapping (both PTE already present
>    and PTE not-present).  These are two different codepaths and I handle
>    both of them in this patch set.
>
> There is no support in this patch set for drivers to mark themselves
> as being VMA lock friendly; they could implement the ->map_pages
> vm_operation, but if they do, they would be the first.  This is probably
> something we want to change at some point in the future, and I've marked
> where to make that change in the code.
>
> Suren, would you mind dropping this into your benchmarking setup and
> seeing if these relatively minor additions make a difference?

Definitely. Thanks for doing this! I'll report once I have some
numbers but first I need to review the patches to understand which
benchmarks could show the benefit.

>
> Matthew Wilcox (Oracle) (6):
>   mm: Make lock_folio_maybe_drop_mmap() VMA lock aware
>   mm: Call wp_page_copy() under the VMA lock
>   mm: Handle shared faults under the VMA lock
>   mm: Handle COW faults under the VMA lock
>   mm: Handle read faults under the VMA lock
>   mm: Handle write faults to RO pages under the VMA lock
>
>  mm/filemap.c | 13 ++++----
>  mm/memory.c  | 93 ++++++++++++++++++++++++++++++++--------------------
>  2 files changed, 65 insertions(+), 41 deletions(-)
>
> --
> 2.40.1
>