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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 (2603:10b6:a03:8c::12) by SJ0PR10MB4733.namprd10.prod.outlook.com (2603:10b6:a03:2ae::12) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3825.20; Wed, 3 Feb 2021 15:32:34 +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 15:32:34 +0000 Subject: Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation To: Pavel Tatashin Cc: Jason Gunthorpe , LKML , linux-mm , Andrew Morton , Vlastimil Babka , Michal Hocko , David Hildenbrand , Oscar Salvador , Dan Williams , Sasha Levin , Tyler Hicks , Joonsoo Kim , Mike Kravetz , Steven Rostedt , Ingo Molnar , Peter Zijlstra , Mel Gorman , Matthew Wilcox , David Rientjes , John Hubbard , Linux Doc Mailing List , Ira Weiny , linux-kselftest@vger.kernel.org, James Morris References: <20210125194751.1275316-1-pasha.tatashin@soleen.com> <20210125194751.1275316-3-pasha.tatashin@soleen.com> <05a66361-214c-2afe-22e4-12862ea1e4e2@oracle.com> From: Joao Martins Message-ID: Date: Wed, 3 Feb 2021 15:32:22 +0000 In-Reply-To: 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; >>> } >>> >> > > Hi Joao, > >> With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> ~32k usecs when >> pinning a 16G hugetlb file. > > Estimate or you actually measured? > It's what I had measured before sending. The ~ is because there's error variance. >> > >> Splitting can only occur on THP right? If so, perhaps we could retain the @step increment > > Yes, I do not think we can split HugePage, only THP. > Right, that's my impression too. >> 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)); > > Yes, this is reasonable optimization. I will submit a follow up patch > against linux-next.