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[209.85.167.52]) by smtp.gmail.com with ESMTPSA id x22sm355265lfe.10.2021.06.22.11.08.12 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Jun 2021 11:08:12 -0700 (PDT) Received: by mail-lf1-f52.google.com with SMTP id p7so37243100lfg.4 for ; Tue, 22 Jun 2021 11:08:12 -0700 (PDT) X-Received: by 2002:a19:7d04:: with SMTP id y4mr3746251lfc.201.1624385292112; Tue, 22 Jun 2021 11:08:12 -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 11:07:56 -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: Matthew Wilcox Cc: 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: imf09.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=Pc4u8ARW; dmarc=none; spf=pass (imf09.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.167.42 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org X-Stat-Signature: 1s9gr3fbopmmbs8fx9q4y8ooh36wn5t9 X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 1242F6000EEF X-HE-Tag: 1624385295-300655 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 11:05 AM Matthew Wilcox wrote: > > Huh? Last I checked, the fault_in_readable actually read a byte from > the page. It has to wait for the read to complete before that can > happen. Yeah, we don't have any kind of async fault-in model. I'm not sure how that would even look. I don't think it would necessarily be *impossible* (special marker in the exception table to let the fault code know that this is a "prepare" fault), but it would be pretty challenging. Linus