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dkim=none (message not signed) header.d=none;linux-foundation.org; dmarc=none action=none header.from=oracle.com; Received: from MWHPR10MB1389.namprd10.prod.outlook.com (2603:10b6:300:21::22) by CO1PR10MB4514.namprd10.prod.outlook.com (2603:10b6:303:9d::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3784.13; Wed, 20 Jan 2021 21:48:07 +0000 Received: from MWHPR10MB1389.namprd10.prod.outlook.com ([fe80::897d:a360:92db:3074]) by MWHPR10MB1389.namprd10.prod.outlook.com ([fe80::897d:a360:92db:3074%5]) with mapi id 15.20.3763.014; Wed, 20 Jan 2021 21:48:07 +0000 Subject: Re: [PATCH v2 2/5] hugetlb: convert page_huge_active() HPageMigratable flag To: Oscar Salvador Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko , Naoya Horiguchi , Muchun Song , David Hildenbrand , Matthew Wilcox , Andrew Morton References: <20210120013049.311822-1-mike.kravetz@oracle.com> <20210120013049.311822-3-mike.kravetz@oracle.com> <20210120095905.GA4752@localhost.localdomain> <20210120100023.GB4752@localhost.localdomain> From: Mike Kravetz Message-ID: <0ef72fcf-7fd6-02f6-6fb5-69a2c5bcf7a4@oracle.com> Date: Wed, 20 Jan 2021 13:48:05 -0800 User-Agent: Mozilla/5.0 (X11; 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By it's name, page_huge_active implied >>> that a huge page was on the active list. However, that is not really >>> what code checking the flag wanted to know. It really wanted to determine >>> if the huge page could be migrated. This happens when the page is actually >>> added the page cache and/or task page table. This is the reasoning behind >>> the name change. >>> >>> The VM_BUG_ON_PAGE() calls in the *_huge_active() interfaces are not >>> really necessary as we KNOW the page is a hugetlb page. Therefore, they >>> are removed. >>> >>> The routine page_huge_active checked for PageHeadHuge before testing the >>> active bit. This is unnecessary in the case where we hold a reference or >>> lock and know it is a hugetlb head page. page_huge_active is also called >>> without holding a reference or lock (scan_movable_pages), and can race with >>> code freeing the page. The extra check in page_huge_active shortened the >>> race window, but did not prevent the race. Offline code calling >>> scan_movable_pages already deals with these races, so removing the check >>> is acceptable. Add comment to racy code. >>> >>> Signed-off-by: Mike Kravetz >> >> Hi Mike, >> >> This comment addresses both this patch and the next one. >> >> Instead of putting the SetHPageMigratable flag spread over the >> allocation paths, would it make more sense to place it in >> alloc_huge_page before returning the page? >> Then we could opencode SetHPageMigratableIfSupported right there. > > and in putback_active_hugepage. Hi Oscar, In Muchun's series of hugetlb bug fixes, Michal asked the same question. https://lore.kernel.org/linux-mm/7e69a55c-d501-6b42-8225-a677f09fb829@oracle.com/ The 'short answer' is that the this would allow a page to be migrated after allocation but before the page fault code adds it to the page cache or page tables. This actually caused bugs in the past. -- Mike Kravetz