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[47.88.44.37]) by mx.google.com with ESMTPS id x22si2735748pgj.271.2019.06.07.11.51.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Jun 2019 11:51:40 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 47.88.44.37 as permitted sender) client-ip=47.88.44.37; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 47.88.44.37 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=01201311R131e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0TTeuMkB_1559933482; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TTeuMkB_1559933482) by smtp.aliyun-inc.com(127.0.0.1); Sat, 08 Jun 2019 02:51:25 +0800 Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility To: Hugh Dickins Cc: Michal Hocko , "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 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> <217fc290-5800-31de-7d46-aa5c0f7b1c75@linux.alibaba.com> From: Yang Shi Message-ID: Date: Fri, 7 Jun 2019 11:51:22 -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 6/7/19 3:57 AM, Hugh Dickins wrote: > On Thu, 6 Jun 2019, Yang Shi wrote: >> On 5/7/19 10:10 AM, Yang Shi wrote: >>> 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. >> [Forgot cc to mailing lists] >> >> Hi guys, >> >> How should we move forward for this one? Make the sysfs knob >> (/sys/kernel/mm/transparent_hugepage/enabled) to be global for both anonymous >> and tmpfs? Or just treat shmem objects separately from anon memory then fix >> the false-negative of THP eligibility by this patch? > Sorry for not getting back to you sooner on this. > > I don't like to drive design by smaps. I agree with the word "mess" used > several times of THP tunings in this thread, but it's too easy to make > that mess worse by unnecessary changes, so I'm very cautious here. > > The addition of "THPeligible" without an "Anon" in its name was > unfortunate. I suppose we're two releases too late to change that. The smaps shows it is anon vma or shmem vma for the most cases. > > Applying process (PR_SET_THP_DISABLE) and mm (MADV_*HUGEPAGE) > limitations to shared filesystem objects doesn't work all that well. The THP eligibility indicator is per vma, it just reports whether THP is eligible for a specific vma. So, I'm supposed it should keep consistent with MMF_DISABLE_THP and MADV_*HUGEPAGE setting. The current implementation in shmem and kuhugepaged also checks these. > > I recommend that you continue to treat shmem objects separately from > anon memory, and just make the smaps "THPeligible" more often accurate. > > Is your v2 patch earlier in this thread the best for that? The v2 patch treats shmem objects separately from anon memory and it makes the "THPeligible" more often accurate. > No answer tonight, I'll re-examine later in the day. > > Hugh