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

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Message-ID: <ZNRYx8GhYftE4Xeb@casper.infradead.org>
Date:   Thu, 10 Aug 2023 04:25:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, 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>,
        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

On Wed, Aug 09, 2023 at 05:23:46PM -0400, Peter Xu wrote:
> 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?

UTSL.

        /*
         * Add all the PTE mappings of those pages mapped by PTE.
         * Limit the loop to folio_nr_pages_mapped()?
         * Perhaps: given all the raciness, that may be a good or a bad idea.
         */