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[47.88.44.37]) by mx.google.com with ESMTPS id u31si5307491pgl.438.2019.04.24.08.47.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 24 Apr 2019 08:47:57 -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=01201311R591e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07487;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0TQ8p1V-_1556120859; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TQ8p1V-_1556120859) by smtp.aliyun-inc.com(127.0.0.1); Wed, 24 Apr 2019 23:47:41 +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: Wed, 24 Apr 2019 08:47:35 -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 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. 4.8 (the first version has shmem THP merged) behaves exactly in the same way. So, I suspect this may be intended behavior. > > What am I missing? > >> ... >> [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. >> >> Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma") >> Cc: Michal Hocko >> Cc: Vlastimil Babka >> Cc: David Rientjes >> Cc: Kirill A. Shutemov >> Signed-off-by: Yang Shi >> --- >> v2: Check VM_NOHUGEPAGE per Michal Hocko >> >> mm/huge_memory.c | 4 ++-- >> mm/shmem.c | 3 +++ >> 2 files changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 165ea46..5881e82 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -67,8 +67,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma) >> { >> if (vma_is_anonymous(vma)) >> return __transparent_hugepage_enabled(vma); >> - if (vma_is_shmem(vma) && shmem_huge_enabled(vma)) >> - return __transparent_hugepage_enabled(vma); >> + if (vma_is_shmem(vma)) >> + return shmem_huge_enabled(vma); >> >> return false; >> } >> diff --git a/mm/shmem.c b/mm/shmem.c >> index 2275a0f..6f09a31 100644 >> --- a/mm/shmem.c >> +++ b/mm/shmem.c >> @@ -3873,6 +3873,9 @@ bool shmem_huge_enabled(struct vm_area_struct *vma) >> loff_t i_size; >> pgoff_t off; >> >> + if ((vma->vm_flags & VM_NOHUGEPAGE) || >> + test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags)) >> + return false; >> if (shmem_huge == SHMEM_HUGE_FORCE) >> return true; >> if (shmem_huge == SHMEM_HUGE_DENY) >>