From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 07E4135CB7A; Fri, 9 Jan 2026 15:24:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767972288; cv=none; b=ghUHQmRFOtAHX6d/3sLSVUVdWYRzwLJr/2mjoBC5eMBQ2pkzJT9a/lhr6XtrnoC/h2Qh4Ee8eaxSe4omZSCbYIh+HgDRsWLA26A8yez4S8/aotWDV2VukWDlJ6P2Tn7vqlHC7TT7T3TDRtqYrPhxrXixoAuWKF1wiyN8wKa4X/0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767972288; c=relaxed/simple; bh=axvy6DYVP2agzRnl6Zhjb8bX4J+s/WNM34OMhZ6lfpA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Dpp9XQMIOdBZ7IlF1LEHGO6mpknJGqQ+Am8Q00y0o4gL3JqrgBxufe6CdaYgWpeSiLh+fJVDyaSCsb9QFvC9Scyig8aSeEncddTJy4qtpsBpTJLaKbaJwoB2ER0yRuO+xjyz7d7k6DhbCTFyVmDIUbyHBvj7+R+8fxgVDpUGDnE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pU/2H1dF; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="pU/2H1dF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C8D52C4CEF1; Fri, 9 Jan 2026 15:24:39 +0000 (UTC) DKIM-Signature: v=1; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 >> 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