From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754266AbaIVOa4 (ORCPT ); Mon, 22 Sep 2014 10:30:56 -0400 Received: from mail-lb0-f176.google.com ([209.85.217.176]:43500 "EHLO mail-lb0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754128AbaIVOay (ORCPT ); Mon, 22 Sep 2014 10:30:54 -0400 MIME-Version: 1.0 In-Reply-To: <20140922101221.4bf46809@lwn.net> References: <20140919104204.3b0bb762@lwn.net> <20140922101221.4bf46809@lwn.net> Date: Mon, 22 Sep 2014 10:30:52 -0400 Message-ID: Subject: Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only) From: Milosz Tanski To: Jonathan Corbet Cc: LKML , Christoph Hellwig , "linux-fsdevel@vger.kernel.org" , linux-aio@kvack.org, Mel Gorman , Volker Lendecke , Tejun Heo , Jeff Moyer , "Theodore Ts'o" , Al Viro Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 22, 2014 at 10:12 AM, Jonathan Corbet wrote: > On Fri, 19 Sep 2014 13:33:14 -0400 > Milosz Tanski wrote: > >> > - Non-blocking I/O has long been supported with a well-understood set >> > of operations - O_NONBLOCK and fcntl(). Why do we need a different >> > mechanism here - one that's only understood in the context of >> > buffered file I/O? I assume you didn't want to implement support >> > for poll() and all that, but is that a good enough reason to add a >> > new Linux-specific non-blocking I/O technique? >> >> I realized that I didn't answer this question well in my other long >> email. O_NONBLOCK doesn't work on files under any commonly used OS, >> and people have gotten use to this behavior so I doubt we could change >> that without breaking a lot of folks applications. > > So I'm not contesting this, but I am genuinely curious: do you think > there are applications out there requesting non-blocking behavior on > regular files that will then break if they actually get non-blocking > behavior? I don't suppose you have an example? > > Thanks, > > jon Earlier in this thread Jeff pointed ( https://lkml.org/lkml/2014/9/15/942 ) to a bug in RH bugzilla ( https://bugzilla.redhat.com/show_bug.cgi?id=136057 ) when an application (squid) reads regular disk files started returning EAGAIN when read from (provided that they were open with O_NONBLOCK) and since that doesn't cause readhead it spins on it forever. As far as I know O_NONBLOCK for regular files in Linux is undefined behavior as non of the man pages I looked at (esp. fnctl, 2 open, 3 open) specify what happens in the case of non-network, non-fifo case (some of them refer to file descriptors that support non-blocking operation, which is pretty vague). So even if squid is wrong in it's behavior (since it's undefined), a quick google search reveals lots of mailing lists / forum posts of people essentially describing the behavior to date. Eg. O_NONBLOCK on regular files blocks, with select/poll/epoll always returning a ready behavior. Based on that anecdotical evidence, I assume a decent chunk of user apps would beak. - Milosz -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@adfin.com