From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03F6BC48BE5 for ; Tue, 22 Jun 2021 16:27:10 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 85EE9611CE for ; Tue, 22 Jun 2021 16:27:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 85EE9611CE Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 686526B0036; Tue, 22 Jun 2021 12:27:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 635336B005D; Tue, 22 Jun 2021 12:27:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4D4AC6B006C; Tue, 22 Jun 2021 12:27:08 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0089.hostedemail.com [216.40.44.89]) by kanga.kvack.org (Postfix) with ESMTP id 1E7C76B0036 for ; Tue, 22 Jun 2021 12:27:08 -0400 (EDT) Received: from smtpin36.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 539AC8249980 for ; Tue, 22 Jun 2021 16:27:08 +0000 (UTC) X-FDA: 78281889336.36.C556F8B Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf13.hostedemail.com (Postfix) with ESMTP id E9F70E002CC7 for ; Tue, 22 Jun 2021 16:27:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1624379227; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Hrhx/d0n+ruCXAaNUiGvt1DI7m7Wkz8aq+QQEOhzPsQ=; b=Jf+RslQb48Q4li6ifQKXXbXGnxXSH6KW9cJKc29cNji/Mmcshezea4PlTrlH7zwqkGVIZQ O+jkBc0hcftvARrOaveGlr/ztCpKLLkLESWg+mhyfaP+Hdx/L72e2qDWz9nSbK8mp5Be+x m9pklfPjo/iHtpjAIKr5si2oMPaM004= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-362-qOaYXXfCMN2sdaBNmGU9jw-1; Tue, 22 Jun 2021 12:27:06 -0400 X-MC-Unique: qOaYXXfCMN2sdaBNmGU9jw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 494929F92E; Tue, 22 Jun 2021 16:27:04 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-118-65.rdu2.redhat.com [10.10.118.65]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0BFBA100760F; Tue, 22 Jun 2021 16:27:01 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <3221175.1624375240@warthog.procyon.org.uk> To: Al Viro Cc: dhowells@redhat.com, torvalds@linux-foundation.org, Ted Ts'o , Dave Hansen , Andrew Morton , willy@infradead.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <3225321.1624379221.1@warthog.procyon.org.uk> Date: Tue, 22 Jun 2021 17:27:01 +0100 Message-ID: <3225322.1624379221@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Jf+RslQb; spf=none (imf13.hostedemail.com: domain of dhowells@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=dhowells@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E9F70E002CC7 X-Stat-Signature: tyqcaki9okmuomu7dkgygc6cfqkt8mti X-HE-Tag: 1624379227-901511 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: Al Viro wrote: > On Tue, Jun 22, 2021 at 04:20:40PM +0100, David Howells wrote: > > > and wondering if the iov_iter_fault_in_readable() is actually effective. > > Yes, it can make sure that the page we're intending to modify is dragged > > into the pagecache and marked uptodate so that it can be read from, but is > > it possible for the page to then get reclaimed before we get to > > iov_iter_copy_from_user_atomic()? a_ops->write_begin() could potentially > > take a long time, say if it has to go and get a lock/lease from a server. > > Yes, it is. So what? We'll just retry. You *can't* take faults while > holding some pages locked; not without shitloads of deadlocks. In that case, can we amend the comment immediately above iov_iter_fault_in_readable()? /* * Bring in the user page that we will copy from _first_. * Otherwise there's a nasty deadlock on copying from the * same page as we're writing to, without it being marked * up-to-date. * * Not only is this an optimisation, but it is also required * to check that the address is actually valid, when atomic * usercopies are used, below. */ if (unlikely(iov_iter_fault_in_readable(i, bytes))) { The first part suggests this is for deadlock avoidance. If that's not true, then this should perhaps be changed. David