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hannes@cmpxchg.org, david@redhat.com, anshuman.khandual@arm.com, linux-mm@kvack.org, Oscar Salvador Subject: Re: [PATCH] mm/page_alloc: fix alignment for alloc_contig_pages_noprof() Date: Tue, 23 Sep 2025 11:05:22 -0400 X-Mailer: MailMate (2.0r6272) Message-ID: <561FB62D-AB2D-4745-B089-59C36A66D27C@nvidia.com> In-Reply-To: References: <20250923001943.2479-1-richard.weiyang@gmail.com> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BN0PR04CA0050.namprd04.prod.outlook.com (2603:10b6:408:e8::25) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|DM6PR12MB4316:EE_ X-MS-Office365-Filtering-Correlation-Id: 614f0945-7a5b-4b5a-6c58-08ddfab2a064 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; X-Microsoft-Antispam-Message-Info: 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List-Subscribe: List-Unsubscribe: On 23 Sep 2025, at 2:47, Dev Jain wrote: > On 23/09/25 5:49 am, 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. >> >> Use roundup() to calculate the correct alignment. >> >> Fixes: 5e27a2df03b8 ("mm/page_alloc: add alloc_contig_pages()") > > The ALIGN() statement was even before this, but not sure if a potential= > caller with non-power-of-2 nr_pages was present then. > >> Signed-off-by: Wei Yang >> Cc: Anshuman Khandual >> Cc: Oscar Salvador >> Cc: Brendan Jackman >> Cc: Johannes Weiner >> Cc: Michal Hocko >> Cc: Suren Baghdasaryan >> Cc: Vlastimil Babka >> Cc: Wei Yang >> Cc: Zi Yan >> --- >> mm/page_alloc.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index a1bcc1e003c7..a17a6014e3db 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -7095,7 +7095,7 @@ struct page *alloc_contig_pages_noprof(unsigned = long nr_pages, gfp_t gfp_mask, >> gfp_zone(gfp_mask), nodemask) { >> spin_lock_irqsave(&zone->lock, flags); >> - pfn =3D ALIGN(zone->zone_start_pfn, nr_pages); >> + pfn =3D roundup(zone->zone_start_pfn, nr_pages); >> while (zone_spans_last_pfn(zone, pfn, nr_pages)) { >> if (pfn_range_valid_contig(pfn, nr_pages)) { >> /* > > The kernel-doc above alloc_contig_pages_noprof reads: > > "If nr_pages is a power of two, then allocated range is always guarante= ed > to be aligned to same nr_pages." This means that the function gives no > guarantee on alignment of non-power-of-2 requests - and why should it? > It doesn't make any sense to align pfn to 7 if nr_pages is 7. I agree with this. But giving a non power of 2 to ALIGN is unexpected, ri= ght? So maybe only use ALIGN for power of 2 nr_pages and leave other cases alo= ne? Or like David suggested in another email, align nr_pages down to power of= 2. Something like: pfn =3D ALIGN(zone->zone_start_pfn, rounddown_pow_of_two(nr_pages)); Best Regards, Yan, Zi