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b=GR9nhZsDiAIoBkK8RJlqM8j3WIVzPu/DIos2PlJB8FUZa6IwT9sVMHNf+DxXGJRsl TVljgZJpbw3njRaecgfLMH7L4FIS8PLfR7+oQ7gtzwr5ynAMtxVl2qyuNyy3ucr/bA 49yp8XMXYF2hP3iBADES7sZ+1t2cVQnAu0wCieoJu4ZGESqNyMW/nP/yypr2PSolxY IMKKsy/NB2B9akd1Di06yKTiWRKK16gU9BSDWGKEq56yNliolY325xpLvfuuNnKghj Nz1XqYsF329+1CdOAzPQN+IVvvDotUvyGd3+03Bv9pU36ouCZy3jFwmAy4NAfbCVoE 0VgDjb1aiyL4Q== Message-ID: <01e5d0b3-dbf8-4f77-b38a-f48c46f7c31e@kernel.org> Date: Fri, 5 Dec 2025 21:44:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization To: Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , Matthew Wilcox , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Usama Arif , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20251205194351.1646318-1-kas@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: xez4m5pw1crx3tkmzri16betgucbpy9o X-Rspam-User: X-Rspamd-Queue-Id: C0EF0180009 X-Rspamd-Server: rspam01 X-HE-Tag: 1764967478-460374 X-HE-Meta: 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 aSnMfIVR yyg2fgYOiEnN67wRXya66b3ljCd4/zkNLGIv8zQA+Eezd72R3wHJMMoQ3toLNI3njRwa0Opwf4XQS0DkeDt8YvBVOH0b375t0ltUTaIzlXeZtWn8H/06JrGTTvoJ7ugOB3MQf67BKOqMfHTQo8PdACxKRkjbHWj5VrLASqmWpWqeoN8+aqC03m7IczgBIwQzHKUHl76uiiVK1TxLmcqBkctRemkmSs77oAAd/NpK7+Dg6ZbI0BZNbcts8v4KuLbSa7eZ9 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 12/5/25 21:33, Kiryl Shutsemau wrote: > On Fri, Dec 05, 2025 at 09:16:08PM +0100, David Hildenbrand (Red Hat) wrote: >> On 12/5/25 20:43, Kiryl Shutsemau wrote: >>> This series removes "fake head pages" from the HugeTLB vmemmap >>> optimization (HVO) by changing how tail pages encode their relationship >>> to the head page. >>> >>> It simplifies compound_head() and page_ref_add_unless(). Both are in the >>> hot path. >>> >>> Background >>> ========== >>> >>> HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages >>> and remapping the freed virtual addresses to a single physical page. >>> Previously, all tail page vmemmap entries were remapped to the first >>> vmemmap page (containing the head struct page), creating "fake heads" - >>> tail pages that appear to have PG_head set when accessed through the >>> deduplicated vmemmap. >>> >>> This required special handling in compound_head() to detect and work >>> around fake heads, adding complexity and overhead to a very hot path. >>> >>> New Approach >>> ============ >>> >>> For architectures/configs where sizeof(struct page) is a power of 2 (the >>> common case), this series changes how position of the head page is encoded >>> in the tail pages. >>> >>> Instead of storing a pointer to the head page, the ->compound_info >>> (renamed from ->compound_head) now stores a mask. >> >> (we're in the merge window) >> >> That doesn't seem to be suitable for the memdesc plans, where we want all >> tail pages do directly point at the allocated memdesc (e.g., struct folio), >> no? > > Sure. My understanding is that it is going to eliminate a need in > compound_head() completely. I don't see the conflict so far. Right. All compound_head pointers will point at the allocated memdesc. Would we still have to detect fake head pages though (at least for some transition period)? I don't recall whether we'll really convert all memdesc users at once, or if some memdescs will co-exist with ordinary compound pages for a while. -- Cheers David