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b=sCGRM+PRyGgOyssVyvdukY1De8h4831Ciolpqds8PUVcS1ysLBcjh7HmmnfiahlQR UEXl9gsl4cXbGZt7Pl9PPyfqHFngUDeiC96ASVObz5Wm3SFr9Vj870kosRSNYWwg6W 3v/P6rUMXfnQiJmBrEVHMshasXlyXWezvOHZAPyfQU0UV8qxGguqtLa+rw6j8PEZjb crNbWr2uHHmV3BAJjRCL4br0SKX5mQf33giCaBsVhYi6Q4BiAldqsMxjZM01v+Yvok HGbiIw+EZRp7gBs6rDtcPQpTSvRfzNHJWuFPHPngRj6oChfnL7YZv7vBEEE4VgTvHN zsskoDr8poN5Q== Message-ID: <6e182cb7-6f32-41c0-ba86-520728b161c7@kernel.org> Date: Thu, 5 Feb 2026 15:09:16 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 07/17] mm: Rework compound_head() for power-of-2 sizeof(struct page) To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-8-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: E3B6040004 X-Stat-Signature: e1uaetgfutcoxbc4bcmjnkfkr8k418sr X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1770300566-329877 X-HE-Meta: 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 iT+3i/qv UyqD36WjPTL6fIqZVG36G10iGx/Un2HMyT9edOxWijBJ3EWlEQY2Xai4tsfteONzVdor2l5fL1pWV/BWEHrK8ZdDqMyYHGAE9Qbm/XHEIXeWRTtuu7Z7jTujNbGm20RT+wp1yrB87HjbUGEx8LUT+GnFRj5/X1tlv+awLplM+GL7orxOd/Id0Tz58r71TyouZOVqDD6SMDSorP5DEe6oH3jxP6jCAhHTqc0OYaNmzhe28O6wvlqDnntEVrPDhX0Si05QIKYBezD++qTH9CyYv73fyqi1zZ8BwY2YItlRZpESwqUUluX1qN52yyf5q7L5GwDpOID/Pjeccb5pJJSmSgbOed4yTWfrU8YVP X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/2/26 16:56, Kiryl Shutsemau wrote: > For tail pages, the kernel uses the 'compound_info' field to get to the > head page. The bit 0 of the field indicates whether the page is a > tail page, and if set, the remaining bits represent a pointer to the > head page. > > For cases when size of struct page is power-of-2, change the encoding of > compound_info to store a mask that can be applied to the virtual address > of the tail page in order to access the head page. It is possible > because struct page of the head page is naturally aligned with regards > to order of the page. > > The significant impact of this modification is that all tail pages of > the same order will now have identical 'compound_info', regardless of > the compound page they are associated with. This paves the way for > eliminating fake heads. > > The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only > applied when the sizeof(struct page) is power-of-2. Having identical > tail pages allows the same page to be mapped into the vmemmap of all > pages, maintaining memory savings without fake heads. > > If sizeof(struct page) is not power-of-2, there is no functional > changes. > > Limit mask usage to HugeTLB vmemmap optimization (HVO) where it makes > a difference. The approach with mask would work in the wider set of > conditions, but it requires validating that struct pages are naturally > aligned for all orders up to the MAX_FOLIO_ORDER, which can be tricky. > > Signed-off-by: Kiryl Shutsemau > Reviewed-by: Muchun Song > Reviewed-by: Zi Yan [...] > struct folio *foliop; > int loops = 5; > > @@ -1247,8 +1247,8 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page) > again: > memset(&ps->folio_snapshot, 0, sizeof(struct folio)); > memcpy(&ps->page_snapshot, page, sizeof(*page)); > - head = ps->page_snapshot.compound_info; > - if ((head & 1) == 0) { > + info = ps->page_snapshot.compound_info; > + if (!(info & 1)) { > ps->idx = 0; > foliop = (struct folio *)&ps->page_snapshot; > if (!folio_test_large(foliop)) { > @@ -1259,7 +1259,15 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page) > } > foliop = (struct folio *)page; > } else { > - foliop = (struct folio *)(head - 1); > + /* See compound_head() */ > + if (compound_info_has_mask()) { > + unsigned long p = (unsigned long)page; > + > + foliop = (struct folio *)(p & info); IIUC, we don't care about clearing bit0 before the & as the page pointer shouldn't have set it in the first page. Pretty neat Acked-by: David Hildenbrand (Arm) -- Cheers, David