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Date:   Fri, 15 Sep 2023 16:40:59 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Daniel Gomez <da.gomez@samsung.com>,
        "minchan@kernel.org" <minchan@kernel.org>,
        "senozhatsky@chromium.org" <senozhatsky@chromium.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        "djwong@kernel.org" <djwong@kernel.org>,
        "hughd@google.com" <hughd@google.com>,
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        "mcgrof@kernel.org" <mcgrof@kernel.org>,
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        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "gost.dev@samsung.com" <gost.dev@samsung.com>,
        Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH 0/6] shmem: high order folios support in write path
Message-ID: <ZQR7CyddIQQAs3yb@casper.infradead.org>
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On Fri, Sep 15, 2023 at 05:36:27PM +0200, David Hildenbrand wrote:
> On 15.09.23 17:34, Matthew Wilcox wrote:
> > No, it can't.  This patchset triggers only on write, not on read or page
> > fault, and it's conservative, so it will only allocate folios which are
> > entirely covered by the write.  IOW this is memory we must allocate in
> > order to satisfy the write; we're just allocating it in larger chunks
> > when we can.
> 
> Oh, good! I was assuming you would eventually over-allocate on the write
> path.

We might!  But that would be a different patchset, and it would be
subject to its own discussion.

Something else I've been wondering about is possibly reallocating the
pages on a write.  This would apply to both normal files and shmem.
If you read in a file one byte at a time, then overwrite a big chunk of
it with a large single write, that seems like a good signal that maybe
we should manage that part of the file as a single large chunk instead
of individual pages.  Maybe.

Lots of things for people who are obsessed with performance to play
with ;-)