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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767972285; bh=axvy6DYVP2agzRnl6Zhjb8bX4J+s/WNM34OMhZ6lfpA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pU/2H1dFfG93kpEUPYu7w7/VXmb/gCiR8gQewSBXnKe7C5c4PF13ehhOoFbpYoyja WDQ5yuTteGs4Ne/v+LHD95mctIe9ZXiABGUWlCzbI96OatJTh7TUC2kQfsTcrOCQ5A MnnrqwAX2OCoFXiTaArGYxiyC+ZVHKSfJpXYF+s6ARfOY1q/4/Ol1fMvuk3t08IIsM WUIczW4dq6yEy6fGrE7HUFUa8zIbX/aucAWzmld+iwpa5dguBZr9Ec5tQvYJoMQgS1 VzubOeshaPJ9CVip+imoA2w5haEZAr9lrvKJ3KXAjL3LjmYBB/16YrhPKpjxL5W8ie Kz6tThsSMaS1A== Message-ID: Date: Fri, 9 Jan 2026 16:24:37 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment To: Kiryl Shutsemau Cc: Matthew Wilcox , Muchun Song , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Andrew Morton , Usama Arif , Frank van der Linden References: <3b758468-9985-49b8-948a-e5837decf52d@kernel.org> <4f82b8ef-77de-422b-a9a5-691c4eca24a3@kernel.org> <2ace6fc2-6891-4d6c-98de-c027da03d516@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: px78qta81996aio9oh66hq1jbagnuxgf X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: BC331C0002 X-HE-Tag: 1767972286-524879 X-HE-Meta: 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 D7/riCzw uEuOWADEEiAT7k8RfxliulhJaUVZiI1eCcCyZP5IroCWGoMlSSrgJshLrpUiUtFScnrnlEh5rtQEMqBMLSK2CD+ZGN1YGFXIaZEnmQo2Mz2xi4h7OLa1nQxqGiWwbcKSeiUbud94j0KZWZwHKkUnwsZgwcqo6y1zKwFI0SaRroSyFHhrrfsWf9FiKxk6T1PnCM2W6E8MILvyJ9O0oe31H1nGdEncgjWJTvqD3cmhTw//QJpqv3DVtBk2TntBnSu9v9mOD 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: >> For quite some time there will be a magical config option that will switch >> between both layouts. I'd assume that things will get more complicated if we >> suddenly have a "compound_head/folio" pointer and a "compound_info" pointer >> at the same time. >> >> But it's really Willy who has the concept in mind as he is very likely right >> now busy writing some of that code. >> >> I'm just the messenger. >> >> :) >> >> [I would hope that Willy could share his thoughts] > > If you or Willy think that this patch will impede memdesc progress, I am > okay not pushing this patchset upstream. I pinged Willy. > > I was really excited when I found this trick to get rid of fake heads. > But ultimately, it is a clean up. I failed to find a performance win I > hoped for. I think it's quite nice as a cleanup, and if we wouldn't have memdescs on the horizon that essentially change the code completely in another direction (having all pages point to a struct folio, not just the tail pages), I wouldn't be bringing this up :) > > Also, I try to understand what 32-byte layout means for fake heads. > _refcount in struct page is going to 0 and refcounting happens on folios. Yes, for folios. > So I wounder if we can all pages identical (no tail pages per se) and > avoid fake heads this way? That's the ultimate goal, yes. Essentially, all pages will point to the memdesc, and there will not be a reason to check for head/fake-head etc. I think initially, the compound-page concept might still co-exist for some memdescs that we won't initially allocate separately. But I don't know the details of that. I know that the transition phase is tricky :) Regarding reference and folios: yes exactly. When trying to get a reference, we'll spot in the memdesc field that this is a folio and try on the folio instead. In the future, most pages will either be permanently frozen and not have a refcount (e.g., struct ptdesc), or have a refcount in their memdesc. In the transition, the location of the refcount depends on memdesc type (in memdesc vs. in page). -- Cheers David