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[109.81.31.43]) by smtp.gmail.com with UTF8SMTPSA id a640c23a62f3a-b2ba267f745sm745653866b.33.2025.09.24.05.01.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Sep 2025 05:01:21 -0700 (PDT) Date: Wed, 24 Sep 2025 14:01:20 +0200 From: Michal Hocko To: David Hildenbrand Cc: Zi Yan , Anshuman Khandual , Dev Jain , Wei Yang , akpm@linux-foundation.org, vbabka@suse.cz, surenb@google.com, jackmanb@google.com, hannes@cmpxchg.org, linux-mm@kvack.org, Oscar Salvador Subject: Re: [PATCH] mm/page_alloc: fix alignment for alloc_contig_pages_noprof() Message-ID: References: <20250923001943.2479-1-richard.weiyang@gmail.com> <3BF1389C-4B8E-4F50-AB0B-F0D96833D387@nvidia.com> <5491dfda-862a-4c31-b64f-5339007a5302@arm.com> <09284390-5a1a-47e0-9929-0a5ba98c836e@redhat.com> <760C8EBD-9201-4680-8B9B-CAD7641A81C4@nvidia.com> <81a827ce-24f7-4038-bca7-c96dc558cb38@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <81a827ce-24f7-4038-bca7-c96dc558cb38@redhat.com> X-Rspamd-Queue-Id: 989644001D X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 7ddeg4nuy48zcaazmua5bf79t4zc5eka X-HE-Tag: 1758715283-873306 X-HE-Meta: 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 FLuzOTn7 3jbZT+ucygOV5nNQMTqrJmfk+x3kpsKCnnG3v6W80fIfv43R/Jv/LwhNLvs3CkAjXWgd9YW4E3vYdeyleNXsNRML08P2DkHXq14s6VdGbuOQ4HL2C3q9bq7exWoh5qiBQ26CpOD1gszko4X1j37L1Mmj3phxHxT5ygDLGgsLhmK3TO5z7JwXjyw4iJ7kfMPuv6qCabaGBmECBayX1fuUPu6WK1xvZYnRqb2il2Bg091LF3EzbTn5t7cmD+l/aEBZEyfKmW9xk7nWp0Q3c5SUNX5oMljcqJ/nd42GDiabJieGEdVB6p/HCTSTnuAGGtpJE/dvrznFfSnFRArFJ5GankPjwpbK5q7HfbkHvWqgiRDNNGvd86Qd0SicxNRnMNp9tOkzpg8h/vCmf55Vr5yj98GfjZHYa2Ousq7+msF76fVDzeKpfeTFSyW2EEQ== 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: On Wed 24-09-25 13:40:58, David Hildenbrand wrote: > On 24.09.25 13:32, Michal Hocko wrote: > > On Tue 23-09-25 11:07:28, Zi Yan wrote: > > > On 23 Sep 2025, at 3:48, David Hildenbrand wrote: > > > > > > > On 23.09.25 09:06, Anshuman Khandual wrote: > > > > > On 23/09/25 7:16 AM, Zi Yan wrote: > > > > > > On 22 Sep 2025, at 20:19, Wei Yang wrote: > > > > > > > > > > > > > Commit 5e27a2df03b8 ("mm/page_alloc: add alloc_contig_pages()") > > > > > > > introduced generic method for alloc_contig_pages(). But the alignment > > > > > > > calculation seems wrong. > > > > > > > > > > > > > > Since ALIGN() only accept power of two value, while nr_pages could be > > > > > > > any positive one, the result is not defined. > > > > > > > > > > > > The result would not be any value lower than zone->zone_start_pfn, > > > > > > so the worst case is getting an unaligned PFN range. I guess > > > > > > most of the time nr_pages would be power of 2. > > > > > > > > > > Agreed. > > > > > > > > > > Also as Dev had pointed out earlier, this function gives no > > > > > guarantee on alignment of non-power-of-2 requests. Hence I > > > > > don't have a strong opinion either way, but does it really > > > > > qualify for a "Fixes:" tag ? > > > > > > > > I'd say if there is nothing to fix, then this patch is not required. > > > > > > > > What likely does make sense would be that a non-power-of-2 would be aligned to the smallest contained power of 2. > > > > > > > > E.g., a 6 MiB request would be aligned to 2 MiB instead of multiples of 6 MiB. > > > > > > > > Not sure what the existing ALIGN would do with that ... > > > > > > Something like the code below: > > > > > > pfn = ALIGN(zone->zone_start_pfn, rounddown_pow_of_two(nr_pages)); > > > > Please let's not try to fix a non-existent problem here. If there is no > > alignement requirement then make sure we document it and drop the > > existing ALIGN which doesn't seem to serve any actual purpose rather > > than make the code more subtle and more obscure for no known reason. > > I think we need at least one initial align because the zone_start_pfn might > not be aligned to the desired order/nr_pages. I thought we have concluded that there is no user expecting a specific alignment. Or have I misunderstood that? -- Michal Hocko SUSE Labs