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Fri, 5 Dec 2025 16:41:19 -0500 (EST) Date: Fri, 5 Dec 2025 21:41:18 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Red Hat)" 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 Subject: Re: [PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization Message-ID: <3v5hdubqnil6w54kimvbgapghj7irjp7xuqma6uxtsrpvj22ph@6t47vsevdwyi> References: <20251205194351.1646318-1-kas@kernel.org> <01e5d0b3-dbf8-4f77-b38a-f48c46f7c31e@kernel.org> <1b659d59-b1c1-4910-baab-0eef7cda234f@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1b659d59-b1c1-4910-baab-0eef7cda234f@kernel.org> X-Stat-Signature: 3basbusndny4ea6dx16or1jq8xa9y7wu X-Rspam-User: X-Rspamd-Queue-Id: 49A0AC000A X-Rspamd-Server: rspam01 X-HE-Tag: 1764970882-822961 X-HE-Meta: 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 OAvwO87u IBw1+kjvQbcFZnBWahHXeThuEmIc7V/maAU+pCGjO4v67XJzDFaBKYuj3ZThg0VgvJDqm2u8HzhvRnKzJ3yWfRVP8v1G93WY9JerbjpM7GNHawC2clxdoGGnbxVpQN3SzswGZMyFUBDYU30NBrJ2+WI6AE3h75yTiAPWLyc0sNbh3h+g0CPvMzcT86ahjYfhbi53qKiepL/T43qf5bjGiZ7nfe7wm0g/RIaTmEgabjQBxbwrqJUDZPxhDH8tWQZ500G7+4bMKo8g3kytniYecR8yOyQ== 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 Fri, Dec 05, 2025 at 10:34:48PM +0100, David Hildenbrand (Red Hat) wrote: > On 12/5/25 21:54, Kiryl Shutsemau wrote: > > On Fri, Dec 05, 2025 at 09:44:30PM +0100, David Hildenbrand (Red Hat) wrote: > > > 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)? > > > > If we need to detect if the memdesc is tail it should be as trivial as > > comparing the given memdesc to the memdesc - 1. If they match, you are > > looking at the tail. > > How could you assume memdesc - 1 exists without performing other checks? Map zero page in front of every discontinuous vmemmap region :P -- Kiryl Shutsemau / Kirill A. Shutemov