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Thu, 20 May 2021 20:00:27 +0000 Subject: Re: [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY To: Mina Almasry Cc: Axel Rasmussen , Peter Xu , Linux-MM , Andrew Morton , open list References: <20210513234309.366727-1-almasrymina@google.com> <09dc0712-48e8-8ba2-f170-4c2febcfff83@oracle.com> From: Mike Kravetz Message-ID: <61c78897-27a2-768b-f4fe-04e24b617ab6@oracle.com> Date: Thu, 20 May 2021 13:00:25 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [50.38.35.18] X-ClientProxiedBy: CO2PR04CA0080.namprd04.prod.outlook.com (2603:10b6:102:1::48) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by CO2PR04CA0080.namprd04.prod.outlook.com (2603:10b6:102:1::48) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4150.23 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=ha2KG046; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=WMPhCutb; spf=none (imf09.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 141.146.126.79) smtp.mailfrom=mike.kravetz@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Rspamd-Server: rspam04 X-Stat-Signature: or8h84d6dz11jefwzt3pjkwojdspz56h X-HE-Tag: 1621540834-734764 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: On 5/20/21 12:21 PM, Mina Almasry wrote: > On Thu, May 20, 2021 at 12:18 PM Mina Almasry wrote: >> >> On Thu, May 13, 2021 at 5:14 PM Mike Kravetz wrote: >>> >>> How about this approach? >>> - Keep the check for hugetlbfs_pagecache_present in hugetlb_mcopy_atomic_pte >>> that you added. That will catch the race where the page was added to >>> the cache before entering the routine. >>> - With the above check in place, we only need to worry about the case >>> where copy_huge_page_from_user fails and we must drop locks. In this >>> case we: >>> - Free the page previously allocated. >>> - Allocate a 'temporary' huge page without consuming reserves. I'm >>> thinking of something similar to page migration. >>> - Drop the locks and let the copy_huge_page_from_user be done to the >>> temporary page. >>> - When reentering hugetlb_mcopy_atomic_pte after dropping locks (the >>> *pagep case) we need to once again check >>> hugetlbfs_pagecache_present. >>> - We then try to allocate the huge page which will consume the >>> reserve. If successful, copy contents of temporary page to newly >>> allocated page. Free temporary page. >>> >>> There may be issues with this, and I have not given it deep thought. It >>> does abuse the temporary huge page concept, but perhaps no more than >>> page migration. Things do slow down if the extra page allocation and >>> copy is required, but that would only be the case if copy_huge_page_from_user >>> needs to be done without locks. Not sure, but hoping that is rare. >> >> Just following up this a bit: I've implemented this approach locally, >> and with it it's passing the test as-is. When I hack the code such >> that the copy in hugetlb_mcopy_atomic_pte() always fails, I run into >> this edge case, which causes resv_huge_pages to underflow again (this >> time permemantly): >> >> - hugetlb_no_page() is called on an index and a page is allocated and >> inserted into the cache consuming the reservation. >> - remove_huge_page() is called on this index and the page is removed from cache. >> - hugetlb_mcopy_atomic_pte() is called on this index, we do not find >> the page in the cache and we trigger this code patch and the copy >> fails. >> - The allocations in this code path seem to double consume the >> reservation and resv_huge_pages underflows. >> >> I'm looking at this edge case to understand why a prior >> remove_huge_page() causes my code to underflow resv_huge_pages. >> > > I should also mention, without a prior remove_huge_page() this code > path works fine, so it seems the fact that the reservation is consumed > before causes trouble, but I'm not sure why yet. > Hi Mina, How about quickly posting the code? I may be able to provide more suggestions if I can see the actual code. -- Mike Kravetz