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[115.124.30.45]) by mx.google.com with ESMTPS id n2si2079684otk.63.2019.06.21.11.23.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Jun 2019 11:24:00 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.45 as permitted sender) client-ip=115.124.30.45; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.45 as permitted sender) smtp.mailfrom=yang.shi@linux.alibaba.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0TUqdGdq_1561141423; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TUqdGdq_1561141423) by smtp.aliyun-inc.com(127.0.0.1); Sat, 22 Jun 2019 02:23:46 +0800 Subject: Re: [PATCH] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind To: Vlastimil Babka , Michal Hocko Cc: akpm@linux-foundation.org, mgorman@techsingularity.net, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Eric Dumazet , "David S. Miller" , netdev@vger.kernel.org References: <1560797290-42267-1-git-send-email-yang.shi@linux.alibaba.com> <20190618130253.GH3318@dhcp22.suse.cz> <20190618182848.GJ3318@dhcp22.suse.cz> <68c2592d-b747-e6eb-329f-7a428bff1f86@linux.alibaba.com> <20190619052133.GB2968@dhcp22.suse.cz> <21a0b20c-5b62-490e-ad8e-26b4b78ac095@suse.cz> <687f4e57-5c50-7900-645e-6ef3a5c1c0c7@linux.alibaba.com> <55eb2ea9-2c74-87b1-4568-b620c7913e17@linux.alibaba.com> <9945a66f-4434-b2a6-63ac-3240ef1d52c9@suse.cz> From: Yang Shi Message-ID: <7d770cda-3f62-f1a2-6f48-529ca71bd6bd@linux.alibaba.com> Date: Fri, 21 Jun 2019 11:23:40 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <9945a66f-4434-b2a6-63ac-3240ef1d52c9@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US 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 6/21/19 4:33 AM, Vlastimil Babka wrote: > On 6/20/19 6:08 PM, Yang Shi wrote: >> >> On 6/20/19 12:18 AM, Vlastimil Babka wrote: >>> On 6/19/19 8:19 PM, Yang Shi wrote: >>>>>>> This is getting even more muddy TBH. Is there any reason that we >>>>>>> have to >>>>>>> handle this problem during the isolation phase rather the migration? >>>>>> I think it was already said that if pages can't be isolated, then >>>>>> migration phase won't process them, so they're just ignored. >>>>> Yes，exactly. >>>>> >>>>>> However I think the patch is wrong to abort immediately when >>>>>> encountering such page that cannot be isolated (AFAICS). IMHO it should >>>>>> still try to migrate everything it can, and only then return -EIO. >>>>> It is fine too. I don't see mbind semantics define how to handle such >>>>> case other than returning -EIO. >>> I think it does. There's: >>> If MPOL_MF_MOVE is specified in flags, then the kernel *will attempt to >>> move all the existing pages* ... If MPOL_MF_STRICT is also specified, >>> then the call fails with the error *EIO if some pages could not be moved* >>> >>> Aborting immediately would be against the attempt to move all. >>> >>>> By looking into the code, it looks not that easy as what I thought. >>>> do_mbind() would check the return value of queue_pages_range(), it just >>>> applies the policy and manipulates vmas as long as the return value is 0 >>>> (success), then migrate pages on the list. We could put the movable >>>> pages on the list by not breaking immediately, but they will be ignored. >>>> If we migrate the pages regardless of the return value, it may break the >>>> policy since the policy will *not* be applied at all. >>> I think we just need to remember if there was at least one page that >>> failed isolation or migration, but keep working, and in the end return >>> EIO if there was such page(s). I don't think it breaks the policy. Once >>> pages are allocated in a mapping, changing the policy is a best effort >>> thing anyway. >> The current behavior is: >> If queue_pages_range() return -EIO (vma is not migratable, ignore other >> conditions since we just focus on page migration), the policy won't be >> set and no page will be migrated. > Ah, I see. IIUC the current behavior is due to your recent commit > a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when > MPOL_MF_STRICT is specified") in order to fix commit 6f4576e3687b > ("mempolicy: apply page table walker on queue_pages_range()"), which > caused -EIO to be not returned enough. But I think you went too far and > instead return -EIO too much. If I look at the code in parent commit of > 6f4576e3687b, I can see in queue_pages_range(): > > if ((flags & MPOL_MF_STRICT) || > ((flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) && > vma_migratable(vma))) { > > err = queue_pages_pgd_range(vma, start, endvma, nodes, > flags, private); > if (err) > break; > } > > and in queue_pages_pte_range(): > > if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) > migrate_page_add(page, private, flags); > else > break; > > So originally, there was no returning of -EIO due to !vma_migratable() - > as long as MPOL_MF_STRICT and MPOL_MF_MOVE* was specified, the code > tried to queue for migration everything it could and didn't ever abort, > AFAICS. And I still think that's the best possible behavior. > >> However, the problem here is the vma might look migratable, but some or >> all the underlying pages are unmovable. So, my patch assumes the vma is >> *not* migratable if at least one page is unmovable. I'm not sure if it >> is possible to have both movable and unmovable pages for the same >> mapping or not, I'm supposed the vma would be split much earlier. >> >> If we don't abort immediately, then we record if there is unmovable >> page, then we could do: >> #1. Still follows the current behavior (then why not abort immediately?) > See above how the current behavior differs from the original one. > >> #2. Set mempolicy then migrate all the migratable pages. But, we may end >> up with the pages on node A, but the policy says node B. Doesn't it >> break the policy? > The policy can already be "broken" (violated is probably better word) by > migrate_pages() failing. If that happens, we don't rollback the migrated > pages and reset the policy back, right? I think the manpage is clear > that MPOL_MF_MOVE is a best-effort. Userspace will know that not > everything was successfully migrated (via -EIO), and can take whatever > steps it deems necessary - attempt rollback, determine which exact > page(s) are violating the policy, etc. I see your point. It makes some sense to me. So, the policy should be set if MPOL_MF_MOVE* is specified even though no page is migrated so that we have consistent behavior for different cases: * vma is not migratable * vma is migratable, but pages are unmovable * vma is migratable, pages are movable, but migrate_pages() fails >