linux-kernel - Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

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Date:   Wed, 9 Aug 2023 17:23:46 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

Hi, David,

Some pure questions below..

On Wed, Aug 09, 2023 at 10:32:56AM +0200, David Hildenbrand wrote:
> Let's track the total mapcount for all large folios in the first subpage.
> 
> The total mapcount is what we actually want to know in folio_mapcount()
> and it is also sufficient for implementing folio_mapped(). This also
> gets rid of any "raceiness" concerns as expressed in
> folio_total_mapcount().

Any more information for that "raciness" described here?

> 
> With sub-PMD THP becoming more important and things looking promising
> that we will soon get support for such anon THP, we want to avoid looping
> over all pages of a folio just to calculate the total mapcount. Further,
> we may soon want to use the total mapcount in other context more
> frequently, so prepare for reading it efficiently and atomically.

Any (perhaps existing) discussion on reduced loops vs added atomic
field/ops?

I had a feeling that there's some discussion behind the proposal of this
patch, if that's the case it'll be great to attach the link in the commit
log.

Thanks,

-- 
Peter Xu