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[115.124.30.45]) by mx.google.com with ESMTPS id f12si20456291pgg.518.2019.05.07.10.10.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 07 May 2019 10:10:46 -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=01201311R901e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04395;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0TR7KsnZ_1557249036; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TR7KsnZ_1557249036) by smtp.aliyun-inc.com(127.0.0.1); Wed, 08 May 2019 01:10:41 +0800 Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility To: Michal Hocko Cc: "Kirill A. Shutemov" , vbabka@suse.cz, rientjes@google.com, kirill@shutemov.name, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins References: <1556037781-57869-1-git-send-email-yang.shi@linux.alibaba.com> <20190423175252.GP25106@dhcp22.suse.cz> <5a571d64-bfce-aa04-312a-8e3547e0459a@linux.alibaba.com> <859fec1f-4b66-8c2c-98ee-2aee9358a81a@linux.alibaba.com> <20190507104709.GP31017@dhcp22.suse.cz> From: Yang Shi Message-ID: Date: Tue, 7 May 2019 10:10:33 -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: <20190507104709.GP31017@dhcp22.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 5/7/19 3:47 AM, Michal Hocko wrote: > [Hmm, I thought, Hugh was CCed] > > On Mon 06-05-19 16:37:42, Yang Shi wrote: >> >> On 4/28/19 12:13 PM, Yang Shi wrote: >>> >>> On 4/23/19 10:52 AM, Michal Hocko wrote: >>>> On Wed 24-04-19 00:43:01, 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 >>>>> ... >>>>> [snip] >>>>> ... >>>>> THPeligible:    0 >>>>> >>>>> And, /proc/meminfo does show THP allocated and PMD mapped too: >>>>> >>>>> ShmemHugePages:     4096 kB >>>>> ShmemPmdMapped:     4096 kB >>>>> >>>>> This doesn't make too much sense.  The anonymous THP flag should not >>>>> intervene shmem THP.  Calling shmem_huge_enabled() with checking >>>>> MMF_DISABLE_THP sounds good enough.  And, we could skip stack and >>>>> dax vma check since we already checked if the vma is shmem already. >>>> Kirill, can we get a confirmation that this is really intended behavior >>>> rather than an omission please? Is this documented? What is a global >>>> knob to simply disable THP system wise? >>> Hi Kirill, >>> >>> Ping. Any comment? >> Talked with Kirill at LSFMM, it sounds this is kind of intended behavior >> according to him. But, we all agree it looks inconsistent. >> >> So, we may have two options: >>     - Just fix the false negative issue as what the patch does >>     - Change the behavior to make it more consistent >> >> I'm not sure whether anyone relies on the behavior explicitly or implicitly >> or not. > Well, I would be certainly more happy with a more consistent behavior. > Talked to Hugh at LSFMM about this and he finds treating shmem objects > separately from the anonymous memory. And that is already the case > partially when each mount point might have its own setup. So the primary > question is whether we need a one global knob to controll all THP > allocations. One argument to have that is that it might be helpful to > for an admin to simply disable source of THP at a single place rather > than crawling over all shmem mount points and remount them. Especially > in environments where shmem points are mounted in a container by a > non-root. Why would somebody wanted something like that? One example > would be to temporarily workaround high order allocations issues which > we have seen non trivial amount of in the past and we are likely not at > the end of the tunel. Shmem has a global control for such use. Setting shmem_enabled to "force" or "deny" would enable or disable THP for shmem globally, including non-fs objects, i.e. memfd, SYS V shmem, etc. > > That being said I would be in favor of treating the global sysfs knob to > be global for all THP allocations. I will not push back on that if there > is a general consensus that shmem and fs in general are a different > class of objects and a single global control is not desirable for > whatever reasons. OK, we need more inputs from Kirill, Hugh and other folks. > > Kirill, Hugh othe folks?