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imf07.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=MIvRBM5m; spf=pass (imf07.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1777099643; a=rsa-sha256; cv=none; b=FvLpBV5q2+XIBePSmAgpwcl00aGPsnAETE537MdGAHdVWod3Wb2/0VVV9RPiMw56Rqfk7e //YHLjgcyP0miR09WrDHNEjct4k096n/fDJebddYRKlvDgxBLOd+VpHCZnFX58HXGFwQUl 7jwn+bEAF+LChrSipdA5r3jWVREzrl0= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 9768A60052; Sat, 25 Apr 2026 06:47:22 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50A28C2BCB0; Sat, 25 Apr 2026 06:47:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777099642; bh=Qdw6w3wNTsj1mPRlJXca8cl67BuW4C+HLS7hqpixnCk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=MIvRBM5mIMO4duMrxyQfXbhFAhzQnVJLFSQ8HzKDddIcbQYD6zxitNjepTmYL02Ly +mkYuMdUD70e/qzuzxhJFCNveB+fL8/kWIo9qTSlAZzEs5MswVqPqe4xX3COd7YjlP CqnEAVW/jsygw6regCWVQU7mqxQy7sl9qnG59FRkR71PecB9RF1N0iuVSKsIdTk+GH EKz6gFV/y3WGwrgbBVK8KWG79TZqeqWx1Png+77ryElIHvU7INDxR2hMSOiNy4uS71 qDTjfvASyJcz0l5y/9zucevkKSb+YdsxNziBEV7VYdRtXsMX5G9QF9xjxwQwPI3Xvz GfyvrJB1amKCg== Message-ID: <2e664019-f161-44d9-a3fa-74c4d8290345@kernel.org> Date: Sat, 25 Apr 2026 08:47:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 4/7] mm/sparse-vmemmap: Fix DAX vmemmap accounting with optimization To: Muchun Song Cc: Muchun Song , Andrew Morton , Oscar Salvador , Michael Ellerman , Madhavan Srinivasan , Lorenzo Stoakes , Liam R Howlett , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Nicholas Piggin , Christophe Leroy , aneesh.kumar@linux.ibm.com, joao.m.martins@oracle.com, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <02e35414-8c30-4753-9403-432d90263f39@kernel.org> <17902B08-7487-4FC8-8EBC-268CE5F3E1B9@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: AE22240011 X-Stat-Signature: 787angmwycf8sfecu8dc9taaaeh6c479 X-HE-Tag: 1777099643-723480 X-HE-Meta: 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 tqhwuBS1 HxYgwsg8Jbh2VWjyjD4XJ0F+PRRJKnpEAGsI91fBVsmCTmCWgfDC7e7dh2hdWNlrJ8Qw8YeJPOhyBT06egJYz0RPpG40v7WHth1OpGvMgWGkRfZ0sgOVgg8glq9Bz3/wJH/gPRV/QZIOLH+AUJI2ez1SXUEX6n08NbJKy6nDS7lp9Wyn6tB4t6PcOov/98qklXrfr8bM9EPG71c+PkgoQhYhnTEOyZfrs/WV3zQZzIC6kxBHhmfw5IcFjiT22VriDgSkTPw0rvjiI0/98GVFwUr3CBEQ/yYhPMnD3lPvdM7hZwjm5WxkT8gXWcg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/25/26 08:20, Muchun Song wrote: > > >> On Apr 25, 2026, at 13:48, David Hildenbrand (Arm) wrote: >> >>  >>> >>> >>> Hi David, >>> >>> Sorry, I missed the 1GB hugepage scenario earlier. Given that sparse_add_section() >>> operates on a scale between PAGES_PER_SUBSECTION and PAGES_PER_SECTION, the pfn and >>> nr_pages parameters wouldn't be aligned with the hugepage size (pages_per_compound), >>> but rather with the PAGES_PER_SECTION boundary. Do you think this explanation makes >>> it clearer? In the interest of code clarity, do you think the modification below >>> makes it easier to follow? >>> >>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c >>> index 2e642c5ff3f2..ce675c5fb94d 100644 >>> --- a/mm/sparse-vmemmap.c >>> +++ b/mm/sparse-vmemmap.c >>> @@ -658,15 +658,18 @@ static int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long n >>> const unsigned int order = pgmap ? pgmap->vmemmap_shift : 0; >>> const unsigned long pages_per_compound = 1UL << order; >>> >>> - VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, >>> - min(pages_per_compound, PAGES_PER_SECTION))); >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION)); >> >> That here makes sense. We can only add/remove in multiples of PAGES_PER_SECTION. >> I think what we are saying is that we want that check in addition to the >> existing min() check. > > Right. > >> >>> VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1)); >>> >>> if (!vmemmap_can_optimize(altmap, pgmap)) >>> return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE); >>> >>> - if (order < PFN_SECTION_SHIFT) >>> + if (order < PFN_SECTION_SHIFT) { >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound)); >>> return VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound; >> >> That makes sense as well, within a section, we expect that we always add/remove >> entire "compound"-managed chunks. >> >>> + } >>> + >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)); >> >> And this is then for the case where a 1G page spans multiple sections, where we >> expect to add/remove an entire section. >> >> So here, indeed the "min" makes sense. I guess we also assume: >> >> VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION); > > Yes. But this one we do not need to explicit it to > assert it since at the front of this function we have > > VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1)); Ah, yes. The alignment checks + VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION); however imply that. So you could simplify by using that check instead of the pfn_to_section_nr() check. But it's still early here ... so whatever you prefer :) -- Cheers, David