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[209.85.208.179]) by smtp.gmail.com with ESMTPSA id x17sm23919ljx.75.2021.06.22.11.57.26 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Jun 2021 11:57:27 -0700 (PDT) Received: by mail-lj1-f179.google.com with SMTP id u20so11864405ljl.13 for ; Tue, 22 Jun 2021 11:57:26 -0700 (PDT) X-Received: by 2002:a2e:22c4:: with SMTP id i187mr4487018lji.251.1624388246482; Tue, 22 Jun 2021 11:57:26 -0700 (PDT) MIME-Version: 1.0 References: <3221175.1624375240@warthog.procyon.org.uk> <3231150.1624384533@warthog.procyon.org.uk> In-Reply-To: From: Linus Torvalds Date: Tue, 22 Jun 2021 11:57:10 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? To: Nadav Amit Cc: Matthew Wilcox , David Howells , Al Viro , "Ted Ts'o" , Dave Hansen , Andrew Morton , Linux-MM , Ext4 Developers List , linux-fsdevel , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=VhTAedbT; spf=pass (imf03.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.167.46 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none X-Stat-Signature: ije8u55iq61qpsmgkry7myxp81muarij X-Rspamd-Queue-Id: BB9E2C0201CC X-Rspamd-Server: rspam06 X-HE-Tag: 1624388250-812451 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 Tue, Jun 22, 2021 at 11:51 AM Nadav Amit wrote: > Just reminding the alternative (in the RFC that I mentioned before): > a vDSO exception table entry for a memory accessing function in the > vDSO. It then behaves as a sort of MADV_WILLNEED for the faulting > page if an exception is triggered. Unlike MADV_WILLNEED it maps the > page if no IO is needed. It can return through a register whether > the page was present or not. Yeah, that looks like a user-space equivalent. And thanks to the vdso, it doesn't need to support all architectures. Unlike a kernel model would (but yes, a kernel model could then have a fallback for the non-prefetching synchronous case instead, so I guess we could just do one architecture at a time). Linus