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a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790667012; bh=fFhSlZBaUkH6xGhsYdrWxCRhPZNn++zm2ii0wkj+7yc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=B4heSQdz/tMGqNY0slQOXLzlZADZOd1dREgw2hLpJuin0+Y/BvyRJVvhE9H311PiN UdkIz6MHgNClCIeaF3ktCwfACFHHvKFYNCNYvebxw9adLLf2IJYvEexDN+mVyzHuAy 3k/ahn482F5vG4IHXX1ST5aRmRIq2QOx/pNZxbLPrW6/fPjaPrD6trp4WhIKWIZb1X xC3QP9ZZFOwLCfdx1AFk10bV8E4GyHCjiFxSjDv3V6OcHCOS7ftwNB4MLeMm7o98nZ sQTK8rhjbfYRyMJVzjNJ/kl1Afn+JBTTLP18zsoD7fwkwmoS6aD7FOlYqJt1FpZkXv jPO0B/TzOk3KA== Message-ID: <751f6586-625b-4485-9f27-409fdf38bf3e@kernel.org> Date: Tue, 29 Sep 2026 09:30:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 06/12] mm/sparse-vmemmap: set compound page order for device DAX To: Muchun Song , Andrew Morton , Oscar Salvador , Madhavan Srinivasan , Michael Ellerman , Jonathan Corbet Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org, Muchun Song , Lorenzo Stoakes , Mike Rapoport , Qi Zheng , Nicholas Piggin , Christophe Leroy , Randy Dunlap , Lance Yang References: <20260927025441.741633-1-songmuchun@bytedance.com> <20260927025441.741633-7-songmuchun@bytedance.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Stat-Signature: d6xxpassup7zdmu5or56h9tnhp6jgyb5 X-Rspam-User: X-Rspamd-Queue-Id: 92E6C8000B X-HE-Tag: 1790667013-267174 X-HE-Meta: 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 DXqoIoyy 1DJqBpuxFebcOTOUVIueBWwOBvp3sm12WIk8iTJOgzbZ4L57Jff8ECasD15vEYp/VDyoO+FxvNErNNgYnme7f4UAlc7Rbd+OGD2cpIFZKudBCHaXi1q7hPGbG9guqgy+XN/6SOs8KajG/EwGEhuPT3OWU6Z2MvF06eb+Gnz/WqrnsUCbpA3mWxfXhOLHb8dV6EMvJeEgPe9DIXd9G6xivV6PVYJpc4M7J4vEwrOz+tGOtzqncF9yIDbNPZv2m2f+U3fBzORTQpZc9reddGLvv3Kp6QVd0usf+RGh42IUINQJvRSD7SjuGJ2ZkL7/JpiD2tMzY Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/27/26 04:54, Muchun Song wrote: > Device DAX can use vmemmap optimization only when a full section is > populated with a compound-page geometry. Record that geometry as the > compound page order in section metadata before populating the section, so > later vmemmap accounting and population decisions can use the section state > directly. > > Clear the compound page order when the section becomes empty again. Also > reject partial additions to a section that already has optimized vmemmap > mappings. compound_nr_pages() determines how many struct pages to > initialize with a section as the smallest granularity. A section therefore > cannot safely mix optimized and ordinary vmemmap layouts. > > Partial additions continue to use ordinary vmemmap population, so they do > not save vmemmap memory. Such additions are uncommon, and the lost saving > is negligible. > > Signed-off-by: Muchun Song > Acked-by: Qi Zheng > --- > v3: > - Update the subject and commit message to use compound page order > terminology > - Use EOPNOTSUPP instead of ENOTSUPP > > v2: > - Explain why optimized and ordinary layouts cannot share a section > (suggested by Qi Zheng) > - Collect Acked-by from Qi Zheng > --- [...]> > static struct page * __meminit section_activate(int nid, unsigned long pfn, > @@ -838,8 +840,13 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn, > struct mem_section *ms = __pfn_to_section(pfn); > struct mem_section_usage *usage = NULL; > struct page *memmap; > + unsigned int order; > int rc; > > + order = vmemmap_can_optimize(altmap, pgmap) ? pgmap->vmemmap_shift : 0; > + if (nr_pages < PAGES_PER_SECTION && section_compound_order(ms)) > + return ERR_PTR(-EOPNOTSUPP); Hm. Why should we support optimizing the vmemmap in case we fall into the same memory section as boot memory? In that case, there already is a memmap allocated during boot for the entire section. IOW, we really shouldn't mess with the vmemmap in case we have an early section. But maybe I am missing something and this is already disallowed? -- Cheers, David