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Shutemov" To: Oleg Nesterov Cc: Song Liu , Linux Kernel Mailing List , Linux MM , Andrew Morton , Matthew Wilcox , "Kirill A. Shutemov" , Kernel Team , William Kucharski , "srikar@linux.vnet.ibm.com" Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP Message-ID: <20190812144045.tkvipsyit3nccvuk@box> References: <20190807233729.3899352-1-songliubraving@fb.com> <20190807233729.3899352-6-songliubraving@fb.com> <20190808163303.GB7934@redhat.com> <770B3C29-CE8F-4228-8992-3C6E2B5487B6@fb.com> <20190809152404.GA21489@redhat.com> <3B09235E-5CF7-4982-B8E6-114C52196BE5@fb.com> <4D8B8397-5107-456B-91FC-4911F255AE11@fb.com> <20190812121144.f46abvpg6lvxwwzs@box> <20190812132257.GB31560@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190812132257.GB31560@redhat.com> User-Agent: NeoMutt/20180716 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 Mon, Aug 12, 2019 at 03:22:58PM +0200, Oleg Nesterov wrote: > On 08/12, Kirill A. Shutemov wrote: > > > > On Fri, Aug 09, 2019 at 06:01:18PM +0000, Song Liu wrote: > > > + if (pte_none(*pte) || !pte_present(*pte)) > > > + continue; > > > > You don't need to check both. Present is never none. > > Agreed. > > Kirill, while you are here, shouldn't retract_page_tables() check > vma->anon_vma (and probably do mm_find_pmd) under vm_mm->mmap_sem? > > Can't it race with, say, do_cow_fault? vma->anon_vma can race, but it doesn't matter. False-negative is fine. It's attempt to avoid taking mmap_sem where it can be not productive. mm_find_pmd() cannot race with do_cow_fault() since the page is locked. __do_fault() has to return locked page before we touch page tables. It is somewhat subtle, but I wanted to avoid taking mmap_sem where it is possible. -- Kirill A. Shutemov