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Date: Fri, 19 Apr 2024 22:02:48 +0800
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Subject: Re: [PATCH v1 04/18] mm: track mapcount of large folios in single
 value
To: David Hildenbrand <david@redhat.com>, <linux-kernel@vger.kernel.org>
CC: <linux-mm@kvack.org>, <linux-doc@vger.kernel.org>,
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	<ryan.roberts@arm.com>, Yang Shi <shy828301@gmail.com>, Zi Yan
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	Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>, "Muchun
 Song" <muchun.song@linux.dev>, Miaohe Lin <linmiaohe@huawei.com>, "Naoya
 Horiguchi" <naoya.horiguchi@nec.com>, Richard Chang <richardycc@google.com>
References: <20240409192301.907377-1-david@redhat.com>
 <20240409192301.907377-5-david@redhat.com>
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From: "Yin, Fengwei" <fengwei.yin@intel.com>
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On 4/10/2024 3:22 AM, 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().
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>