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Wed, 23 Nov 2022 04:45:38 +0000 Received: from BYAPR12MB3176.namprd12.prod.outlook.com ([fe80::ecfb:a3ad:3efa:9df8]) by BYAPR12MB3176.namprd12.prod.outlook.com ([fe80::ecfb:a3ad:3efa:9df8%3]) with mapi id 15.20.5834.015; Wed, 23 Nov 2022 04:45:38 +0000 References: <20221123005752.161003-1-gshan@redhat.com> User-agent: mu4e 1.8.10; emacs 28.2 From: Alistair Popple To: Gavin Shan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, william.kucharski@oracle.com, ziy@nvidia.com, kirill.shutemov@linux.intel.com, david@redhat.com, zhenyzha@redhat.com, shan.gavin@gmail.com Subject: Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation Date: Wed, 23 Nov 2022 15:26:25 +1100 In-reply-to: <20221123005752.161003-1-gshan@redhat.com> Message-ID: <87sfias2hd.fsf@nvidia.com> Content-Type: text/plain X-ClientProxiedBy: SJ0PR03CA0331.namprd03.prod.outlook.com (2603:10b6:a03:39c::6) To BYAPR12MB3176.namprd12.prod.outlook.com (2603:10b6:a03:134::26) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR12MB3176:EE_|CO6PR12MB5443:EE_ X-MS-Office365-Filtering-Correlation-Id: b2c8547c-5273-4aa3-40b9-08dacd0d915b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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b=ZP0wyTfeqPZ+v+1yEErOtdKY/FeIPwXNJCY98+B3rB4igcMJw7fP56vBHuXjLbkz3LOvOl m5nTrPJzV4jxMLPJASgiqFKLsNrVsgDas90o/MQvDuXiaVQcIHJdJMcQtfAAgXBF5DIEtw gIl7U9ExUK2suUygbscVl50xZw0tXWg= X-Stat-Signature: 4s9x11fftegiu6ngw3mepimu8or3ysgn X-Rspamd-Queue-Id: 8A6CC1C000D X-Rspam-User: Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=HGpgdecm; spf=pass (imf21.hostedemail.com: domain of apopple@nvidia.com designates 40.107.237.53 as permitted sender) smtp.mailfrom=apopple@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1") X-Rspamd-Server: rspam02 X-HE-Tag: 1669178740-272152 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: Gavin Shan writes: > The issue is reported when removing memory through virtio_mem device. > The transparent huge page, experienced copy-on-write fault, is wrongly > regarded as pinned. The transparent huge page is escaped from being > isolated in isolate_migratepages_block(). The transparent huge page > can't be migrated and the corresponding memory block can't be put > into offline state. > > Fix it by replacing page_mapcount() with total_mapcount(). With this, > the transparent huge page can be isolated and migrated, and the memory > block can be put into offline state. > > Fixes: 3917c80280c9 ("thp: change CoW semantics for anon-THP") > Cc: stable@vger.kernel.org # v5.8+ > Reported-by: Zhenyu Zhang > Suggested-by: David Hildenbrand > Signed-off-by: Gavin Shan > --- > mm/compaction.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index c51f7f545afe..c408b5e04c1d 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -990,7 +990,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > * admittedly racy check. > */ > mapping = page_mapping(page); > - if (!mapping && page_count(page) > page_mapcount(page)) > + if (!mapping && page_count(page) > total_mapcount(page)) We have several versions of these checks for pinned pages open-coded around the place. See for example migrate_vma_check_page() and folio_expected_refs(). It looks like you could use a variant of migrate_vma_check_page() which would also check non-anon pins, although I don't know the compaction code well enough to know if that's useful. Either way it would be nice if we had a common helper for these kind of checks. Guess that would be harder to backport, and the change itself looks ok. But why isn't the fixes tag 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages")? > goto isolate_fail; > > /*