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 A769EC49EA2 for ; Tue, 22 Jun 2021 16:27:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F73361289 for ; Tue, 22 Jun 2021 16:27:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230436AbhFVQ30 (ORCPT ); Tue, 22 Jun 2021 12:29:26 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:32335 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229718AbhFVQ30 (ORCPT ); Tue, 22 Jun 2021 12:29:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1624379229; 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=bw/OEVu8FaWunGXt/cJf7r3ZnjxT4ccG16LfRqY87B8Tgf8esVVhR1iER0cOd7ECzvc1it pD7fUhbRqDIPmDIm5Z8nVxZVePKNq2i8z/eW7q/7w7OA3C06Yb9EQsD+Dqz9Px82IF1tJ1 Ppj/vJW0kiSQhJNa3aMWT9lpZBvzdWs= 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 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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