From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers Date: Wed, 2 May 2012 13:10:04 +1000 Message-ID: References: <1335778207-6511-1-git-send-email-jack@suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: In-Reply-To: Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Hugh Dickins Cc: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, KOSAKI Motohiro , Jan Kara , LKML , linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, mgorman-l3A5Bk7waGM@public.gmane.org, Jeff Moyer List-Id: linux-man@vger.kernel.org On 2 May 2012 13:04, Hugh Dickins wrote: > On Wed, 2 May 2012, Nick Piggin wrote: >> On 2 May 2012 03:56, Michael Kerrisk (man-pages) wrote: >> > >> > In the light of all of the comments, can someone revise the man-pages >> > patch that Jan sent? >> >> This does not quite describe the entire situation, but something understandable >> to developers: >> >> 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). Yes of course, I forgot that was exported too. > >> >> >> >> Is that on the right track? I feel it might be necessary to describe this >> allowance for MAP_SHARED, because some databases may be doing >> such things, and anyway it gives apps a potential way to make this work >> if concurrent fork + DIO is very important. > > Looks good, but we do need a reference to MADV_DONTFORK, perhaps as above. Yep, thanks Hugh. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html