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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 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list 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 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