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[115.124.30.132]) by mx.google.com with ESMTPS id w10si4476481pgr.296.2019.04.25.09.44.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Apr 2019 09:44:14 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.132 as permitted sender) client-ip=115.124.30.132; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.132 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=01201311R471e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04391;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0TQE6Yfd_1556210649; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TQE6Yfd_1556210649) by smtp.aliyun-inc.com(127.0.0.1); Fri, 26 Apr 2019 00:44:12 +0800 Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility To: Vlastimil Babka , mhocko@suse.com, rientjes@google.com, kirill@shutemov.name, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org__handle_mm_fault References: <1556037781-57869-1-git-send-email-yang.shi@linux.alibaba.com> From: Yang Shi Message-ID: Date: Thu, 25 Apr 2019 09:44:06 -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: 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 4/24/19 9:17 AM, Vlastimil Babka wrote: > On 4/24/19 5:47 PM, Yang Shi wrote: >> >> On 4/24/19 6:10 AM, Vlastimil Babka wrote: >>> On 4/23/19 6:43 PM, Yang Shi wrote: >>>> The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each >>>> vma") introduced THPeligible bit for processes' smaps. But, when checking >>>> the eligibility for shmem vma, __transparent_hugepage_enabled() is >>>> called to override the result from shmem_huge_enabled(). It may result >>>> in the anonymous vma's THP flag override shmem's. For example, running a >>>> simple test which create THP for shmem, but with anonymous THP disabled, >>>> when reading the process's smaps, it may show: >>>> >>>> 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test >>>> Size: 4096 kB >>>> ... >>>> [snip] >>>> ... >>>> ShmemPmdMapped: 4096 kB >>> But how does this happen in the first place? >>> In __handle_mm_fault() we do: >>> >>> if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) { >>> ret = create_huge_pmd(&vmf); >>> if (!(ret & VM_FAULT_FALLBACK)) >>> return ret; >>> >>> And __transparent_hugepage_enabled() checks the global THP settings. >>> If THP is not enabled / is only for madvise and the vma is not madvised, >>> then this should fail, and also khugepaged shouldn't either run at all, >>> or don't do its job for such non-madvised vma. >> If __transparent_hugepage_enabled() returns false, the code will not >> reach create_huge_pmd() at all. If it returns true, create_huge_pmd() >> actually will return VM_FAULT_FALLBACK for shmem since shmem doesn't >> have huge_fault (or pmd_fault in earlier versions) method. >> >> Then it will get into handle_pte_fault(), finally shmem_fault() is >> called, which allocates THP by checking some global flag (i.e. >> VM_NOHUGEPAGE and MMF_DISABLE_THP) and  shmem THP knobs. > Aha, thanks! What a mess... Yes, it does look convoluted. I'm wondering we may consider refactor the shmem THP fault handling. > >> 4.8 (the first version has shmem THP merged) behaves exactly in the same >> way. So, I suspect this may be intended behavior. > Still looks like an oversight to me. And it's inconsistent... it might > fault huge shmem pages when THPs are globally disabled, but khugepaged > is still not running. I think it should just check the global THP flags > as well... It does looks inconsistent, particularly for the khugepaged part.