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wangkefeng.wang@huawei.com, linux-mm@kvack.org, Oscar Salvador Subject: Re: [PATCH] mm/compaction: fix low_pfn advance on isolating hugetlb Date: Thu, 11 Sep 2025 12:19:34 -0400 X-Mailer: MailMate (2.0r6272) Message-ID: <3DE28F4B-ACB1-468F-89B9-D7750D24BE4E@nvidia.com> In-Reply-To: <20250911032751.khtgvdhcqzyf3rgr@master> References: <20250910092240.3981-1-richard.weiyang@gmail.com> <20250911012521.4p7kmxv46kwz5fz5@master> <5F7DCC9D-4CA2-4BA2-9EA8-F04C3883E289@nvidia.com> <20250911032751.khtgvdhcqzyf3rgr@master> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR03CA0013.namprd03.prod.outlook.com (2603:10b6:208:23a::18) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|DS0PR12MB6487:EE_ X-MS-Office365-Filtering-Correlation-Id: 51b03750-fe0f-4a20-cd5d-08ddf14f0028 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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 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 10 Sep 2025, at 23:27, Wei Yang wrote: > On Wed, Sep 10, 2025 at 09:35:53PM -0400, Zi Yan wrote: >> On 10 Sep 2025, at 21:25, Wei Yang wrote: >> >>> On Wed, Sep 10, 2025 at 09:22:40AM +0000, Wei Yang wrote: >>>> Commit 56ae0bb349b4 ("mm: compaction: convert to use a folio in >>>> isolate_migratepages_block()") converts api from page to folio. But = the >>>> low_pfn advance for hugetlb page seems wrong when low_pfn doesn't po= int >>>> to head page. >>>> >>>> Originally, if page is a hugetlb tail page, compound_nr() return 1, >>>> which means low_pfn only advance one in next iteration. After the >>>> change, low_pfn would advance more than the hugetlb range, since >>>> folio_nr_pages() always return total number of the large page. This >>>> results in skipping some range to isolate and then to migrate. >>>> >>>> The worst case for alloc_contig is it does all the isolation and >>>> migration, but finally find some range is still not isolated. And th= en >>>> undo all the work and try a new range. >>>> >>>> Advance low_pfn to the end of hugetlb. >>>> >>>> Signed-off-by: Wei Yang >>>> Fixes: 56ae0bb349b4 ("mm: compaction: convert to use a folio in isol= ate_migratepages_block()") >>>> Cc: Kefeng Wang >>>> Cc: Oscar Salvador >>> >>> Forgot to cc stable. >>> >>> Cc: >> >> Is there any bug report to justify the backport? Since it is more like= ly >> to be a performance issue instead of a correctness issue. >> > > OK, I thought cc-stable is paired with fixes tag. > > If not, please drop it. > >>> >>>> --- >>>> mm/compaction.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/mm/compaction.c b/mm/compaction.c >>>> index bf021b31c7ec..1e8f8eca318c 100644 >>>> --- a/mm/compaction.c >>>> +++ b/mm/compaction.c >>>> @@ -989,7 +989,7 @@ isolate_migratepages_block(struct compact_contro= l *cc, unsigned long low_pfn, >>>> * Hugepage was successfully isolated and placed >>>> * on the cc->migratepages list. >>>> */ >>>> - low_pfn +=3D folio_nr_pages(folio) - 1; >>>> + low_pfn +=3D folio_nr_pages(folio) - folio_page_idx(folio, page= ) - 1; >>> >>> One question is why we advance compound_nr() in original version. >>> >>> Yes, there are several places advancing compound_nr(), but it seems t= o iterate >>> on the same large page and do the same thing and advance 1 again. >>> >>> Not sure which part story I missed. >> >> isolate_migratepages_block() starts from the beginning of a pageblock.= >> How likely the code hit in the middle of a hugetlb? >> > > OK, this is a kind of optimization based on the knowledge it is not lik= ely to > be a tail page? No, it might be that most of the time page is the head, or people assume = so. -- Best Regards, Yan, Zi