linux-kernel - Re: Mapcount of subpages

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Message-ID: <20210924230505.vlql2mdjfcotkuqj@box.shutemov.name>
Date:   Sat, 25 Sep 2021 02:05:05 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: Mapcount of subpages

On Thu, Sep 23, 2021 at 02:10:13PM -0700, Hugh Dickins wrote:
> But you have a different point in mind when you refer to superfluous
> CoW and GUP: I don't know the score there (and I think we are still in
> that halfway zone, since pte CoW was changed to depend on page_count,
> but THP CoW still depending on mapcount).

I didn't pay enough attention to the topic when the change to depend on
page_count was made. I need to catch up.

I look at what direction Andrea went in his patchset and so far I *feel*
he has a point[1]. I have not read the whole thing yet and I don't have a
firm position here, but maybe we need to get to the bottom of the topic
before considering ditching per-subpage mapcount.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/tree/mm/memory.c?h=mapcount_deshare&id=e1cb3108d4131c2a7da03fbd37c3230cf082bfd9#n3153

-- 
 Kirill A. Shutemov