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dkim=none (message not signed) header.d=none;namei.org; dmarc=none action=none header.from=oracle.com; Received: from BYAPR10MB3077.namprd10.prod.outlook.com (20.177.226.140) by BYAPR10MB3093.namprd10.prod.outlook.com (20.179.154.146) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3805.19; Wed, 3 Feb 2021 13:22:31 +0000 Received: from BYAPR10MB3077.namprd10.prod.outlook.com ([fe80::74a8:8649:e20b:d571]) by BYAPR10MB3077.namprd10.prod.outlook.com ([fe80::74a8:8649:e20b:d571%7]) with mapi id 15.20.3805.024; Wed, 3 Feb 2021 13:22:31 +0000 Subject: Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation To: Pavel Tatashin , jgg@ziepe.ca References: <20210125194751.1275316-1-pasha.tatashin@soleen.com> <20210125194751.1275316-3-pasha.tatashin@soleen.com> Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, vbabka@suse.cz, mhocko@suse.com, david@redhat.com, osalvador@suse.de, dan.j.williams@intel.com, sashal@kernel.org, tyhicks@linux.microsoft.com, iamjoonsoo.kim@lge.com, mike.kravetz@oracle.com, rostedt@goodmis.org, mingo@redhat.com, peterz@infradead.org, mgorman@suse.de, willy@infradead.org, rientjes@google.com, jhubbard@nvidia.com, linux-doc@vger.kernel.org, ira.weiny@intel.com, linux-kselftest@vger.kernel.org, jmorris@namei.org From: Joao Martins Message-ID: <05a66361-214c-2afe-22e4-12862ea1e4e2@oracle.com> Date: Wed, 3 Feb 2021 13:22:18 +0000 In-Reply-To: <20210125194751.1275316-3-pasha.tatashin@soleen.com> Content-Type: text/plain; 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However, as Jason noted, it is > not necessary correct that pages[i] corresponds to the pages that > we skipped. This is because it is possible that the addresses in > this range had split_huge_pmd()/split_huge_pud(), and these functions > do not update the compound page metadata. > > The problem can be reproduced if something like this occurs: > > 1. User faulted huge pages. > 2. split_huge_pmd() was called for some reason > 3. User has unmapped some sub-pages in the range > 4. User tries to longterm pin the addresses. > > The resulting pages[i] might end-up having pages which are not compound > size page aligned. > > Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page") > Reported-by: Jason Gunthorpe > Signed-off-by: Pavel Tatashin > Reviewed-by: Jason Gunthorpe > --- [...] > /* > * If we get a page from the CMA zone, since we are going to > * be pinning these entries, we might as well move them out > @@ -1599,8 +1596,6 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm, > } > } > } > - > - i += step; > } > With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> 32k usecs when pinning a 16G hugetlb file. Splitting can only occur on THP right? If so, perhaps we could retain the @step increment for compound pages but when !is_transparent_hugepage(head) or just PageHuge(head) like: + if (!is_transparent_hugepage(head) && PageCompound(page)) + i += (compound_nr(head) - (pages[i] - head)); Or making specific to hugetlbfs: + if (PageHuge(head)) + i += (compound_nr(head) - (pages[i] - head));