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

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Date:   Thu, 10 Aug 2023 22:48:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Ryan Roberts <ryan.roberts@....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>,
        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 Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
> AFAICS if that patch was all correct (while I'm not yet sure..), you can
> actually fit your new total mapcount field into page 1 so even avoid the
> extra cacheline access.  You can have a look: the trick is refcount for
> tail page 1 is still seems to be free on 32 bits (if that was your worry
> before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.

No, refcount must be 0 on all tail pages.  We rely on this in many places
in the MM.