linux-kernel - Re: [PATCH v1 04/18] mm: track mapcount of large folios in single value

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Date: Tue, 09 Apr 2024 16:13:35 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, cgroups@...r.kernel.org, linux-sh@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Peter Xu <peterx@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Yin Fengwei <fengwei.yin@...el.com>,
 Yang Shi <shy828301@...il.com>, Jonathan Corbet <corbet@....net>,
 Hugh Dickins <hughd@...gle.com>, Yoshinori Sato <ysato@...rs.sourceforge.jp>,
 Rich Felker <dalias@...c.org>,
 John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
 Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>,
 Muchun Song <muchun.song@...ux.dev>, Miaohe Lin <linmiaohe@...wei.com>,
 Naoya Horiguchi <naoya.horiguchi@....com>,
 Richard Chang <richardycc@...gle.com>
Subject: Re: [PATCH v1 04/18] mm: track mapcount of large folios in single
 value

On 9 Apr 2024, at 15:22, David Hildenbrand wrote:

> Let's track the mapcount of large folios in a single value. The mapcount of
> a large folio currently corresponds to the sum of the entire mapcount and
> all page mapcounts.
>
> This sum is what we actually want to know in folio_mapcount() and it is
> also sufficient for implementing folio_mapped().
>
> With PTE-mapped THP becoming more important and more widely used, we want
> to avoid looping over all pages of a folio just to obtain the mapcount
> of large folios. The comment "In the common case, avoid the loop when no
> pages mapped by PTE" in folio_total_mapcount() does no longer hold for
> mTHP that are always mapped by PTE.
>
> Further, we are planning on using folio_mapcount() more
> frequently, and might even want to remove page mapcounts for large
> folios in some kernel configs. Therefore, allow for reading the mapcount of
> large folios efficiently and atomically without looping over any pages.
>
> Maintain the mapcount also for hugetlb pages for simplicity. Use the new
> mapcount to implement folio_mapcount() and folio_mapped(). Make
> page_mapped() simply call folio_mapped(). We can now get rid of
> folio_large_is_mapped().
>
> _nr_pages_mapped is now only used in rmap code and for debugging
> purposes. Keep folio_nr_pages_mapped() around, but document that its use
> should be limited to rmap internals and debugging purposes.
>
> This change implies one additional atomic add/sub whenever
> mapping/unmapping (parts of) a large folio.
>
> As we now batch RMAP operations for PTE-mapped THP during fork(),
> during unmap/zap, and when PTE-remapping a PMD-mapped THP, and we adjust
> the large mapcount for a PTE batch only once, the added overhead in the
> common case is small. Only when unmapping individual pages of a large folio
> (e.g., during COW), the overhead might be bigger in comparison, but it's
> essentially one additional atomic operation.
>
> Note that before the new mapcount would overflow, already our refcount
> would overflow: each mapping requires a folio reference. Extend the
> focumentation of folio_mapcount().

s/focumentation/documentation/  ;)

--
Best Regards,
Yan, Zi

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