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[195.135.220.15]) by mx.google.com with ESMTPS id h18si7277078ede.194.2019.04.24.06.13.31 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 24 Apr 2019 06:13:31 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id D32E8AE8D; Wed, 24 Apr 2019 13:13:30 +0000 (UTC) Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility To: Yang Shi , 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: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <1556037781-57869-1-git-send-email-yang.shi@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit 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 4/23/19 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. 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) >