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[209.85.128.51]) by smtp.gmail.com with ESMTPSA id r3sm4698855ejr.79.2021.12.19.13.54.03 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 19 Dec 2021 13:54:04 -0800 (PST) Received: by mail-wm1-f51.google.com with SMTP id z206so5684467wmc.1 for ; Sun, 19 Dec 2021 13:54:03 -0800 (PST) X-Received: by 2002:a1c:7312:: with SMTP id d18mr3505400wmb.8.1639950832965; Sun, 19 Dec 2021 13:53:52 -0800 (PST) MIME-Version: 1.0 References: <5CA1D89F-9DDB-4F91-8929-FE29BB79A653@vmware.com> <4D97206A-3B32-4818-9980-8F24BC57E289@vmware.com> <5A7D771C-FF95-465E-95F6-CD249FE28381@vmware.com> In-Reply-To: From: Linus Torvalds Date: Sun, 19 Dec 2021 13:53:36 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb) To: Matthew Wilcox Cc: David Hildenbrand , Nadav Amit , Jason Gunthorpe , Linux Kernel Mailing List , Andrew Morton , Hugh Dickins , David Rientjes , Shakeel Butt , John Hubbard , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Vlastimil Babka , Jann Horn , Michal Hocko , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Linux-MM , "open list:KERNEL SELFTEST FRAMEWORK" , "open list:DOCUMENTATION" Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: A83F020018 X-Stat-Signature: 1qtsc7ejbj554c3dhmxyyo79h5rcumui Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=gsjwrXj2; spf=pass (imf31.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.208.41 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none X-HE-Tag: 1639950835-237333 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 Sun, Dec 19, 2021 at 1:48 PM Matthew Wilcox wrote: > > Yes, agreed, I was thinking that we could use "not mapped at all" > as an optimisation to avoid doing rmap walks. eg __unmap_and_move(). So the thing is, it's a very dodgy optimization for a rather simple reason: what if somebody pages the page in? So even "not mapped at all" is questionable. You have to check that it's also not a swapcache page, and hold the page lock for that check, at the very least. And by then, you're really in a very unusual situation - and my gut feel says not one worth optimizing for (because anon pages are _usually_ mapped at least once). But I dunno - it might depend on your load. Maybe you have some very special load that happens to trigger this case a lot? Linus