From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755542Ab2EIHBo (ORCPT ); Wed, 9 May 2012 03:01:44 -0400 Received: from mail-ob0-f174.google.com ([209.85.214.174]:35355 "EHLO mail-ob0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755204Ab2EIHBl (ORCPT ); Wed, 9 May 2012 03:01:41 -0400 MIME-Version: 1.0 In-Reply-To: References: <20120502081705.GB16976@quack.suse.cz> <20120502091837.GC16976@quack.suse.cz> <20120502192325.GA18339@quack.suse.cz> Date: Wed, 9 May 2012 17:01:40 +1000 Message-ID: Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers From: Nick Piggin To: mtk.manpages@gmail.com Cc: KOSAKI Motohiro , Jan Kara , Jeff Moyer , LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de, Andrea Arcangeli , Woodman 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 9 May 2012 15:35, Michael Kerrisk (man-pages) wrote: > On Wed, May 9, 2012 at 11:10 AM, Nick Piggin wrote: >> On 6 May 2012 01:29, KOSAKI Motohiro wrote: >>>> So, am I correct to assume that right text to add to the page is as below? >>>> >>>> Nick, can you clarify what you mean by "quiesced"? >>> >>> finished? >> >> Yes exactly. That might be a simpler word. Thanks! > > Thanks. > > But see below. I realize the text is still ambiguous. > >>>> [[ >>>> O_DIRECT IOs should never be run concurrently with fork(2) system call, >>>> when the memory buffer is anonymous memory, or comes from mmap(2) >>>> with MAP_PRIVATE. >>>> >>>> Any such IOs, whether submitted with asynchronous IO interface or from >>>> another thread in the process, should be quiesced before fork(2) is called. >>>> Failure to do so can result in data corruption and undefined behavior in >>>> parent and child processes. >>>> >>>> This restriction does not apply when the memory buffer for the O_DIRECT >>>> IOs comes from mmap(2) with MAP_SHARED or from shmat(2). >>>> Nor does this restriction apply when the memory buffer has been advised >>>> as MADV_DONTFORK with madvise(2), ensuring that it will not be available >>>> to the child after fork(2). >>>> ]] > > In the above, the status of a MAP_SHARED MAP_ANONYMOUS buffer is > unclear. The first paragraph implies that such a buffer is unsafe, > while the third paragraph implies that it *is* safe, thus > contradicting the first paragraph. Which is correct? Yes I see. It's because MAP_SHARED | MAP_ANONYMOUS isn't *really* anonymous from the virtual memory subsystem's point of view. But that just serves to confuse userspace I guess. Anything with MAP_SHARED, shmat, or MADV_DONTFORK is OK. Anything else (MAP_PRIVATE, brk, without MADV_DONTFORK) is dangerous. These type are used by standard heap allocators malloc, new, etc.