linux-kernel - Re: [PATCH 0/6] shmem: high order folios support in write path

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Date:   Fri, 15 Sep 2023 17:43:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Daniel Gomez <da.gomez@...sung.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "gost.dev@...sung.com" <gost.dev@...sung.com>,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH 0/6] shmem: high order folios support in write path

On 15.09.23 17:40, Matthew Wilcox wrote:
> 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 ;-)

:) Absolutely. ... because if nobody will be consuming that written 
memory any time soon, it might also be the wrong place for a large/huge 
folio.

-- 
Cheers,

David / dhildenb