From: Jan Kara <jack@suse.cz>
To: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
linux-man@vger.kernel.org, linux-mm@kvack.org,
Jan Kara <jack@suse.cz>,
mgorman@suse.de, Jeff Moyer <jmoyer@redhat.com>
Subject: [PATCH] Describe race of direct read and fork for unaligned buffers
Date: Mon, 30 Apr 2012 11:30:07 +0200 [thread overview]
Message-ID: <1335778207-6511-1-git-send-email-jack@suse.cz> (raw)
This is a long standing problem (or a surprising feature) in our implementation
of get_user_pages() (used by direct IO). Since several attempts to fix it
failed (e.g.
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg06542.html, or
http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html refused in
http://comments.gmane.org/gmane.linux.kernel.mm/31569) and it's not completely
clear whether we really want to fix it given the costs, let's at least document
it.
CC: mgorman@suse.de
CC: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
--- a/man2/open.2 2012-04-27 00:07:51.736883092 +0200
+++ b/man2/open.2 2012-04-27 00:29:59.489892980 +0200
@@ -769,7 +769,12 @@
and the file offset must all be multiples of the logical block size
of the file system.
Under Linux 2.6, alignment to 512-byte boundaries
-suffices.
+suffices. However, if the user buffer is not page aligned and direct read
+runs in parallel with a
+.BR fork (2)
+of the reader process, it may happen that the read data is split between
+pages owned by the original process and its child. Thus effectively read
+data is corrupted.
.LP
The
.B O_DIRECT
next reply other threads:[~2012-04-30 9:30 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-30 9:30 Jan Kara [this message]
2012-04-30 13:41 ` [PATCH] Describe race of direct read and fork for unaligned buffers Jeff Moyer
2012-04-30 14:30 ` Mel Gorman
2012-05-01 5:50 ` Michael Kerrisk (man-pages)
2012-05-01 6:49 ` Nick Piggin
2012-05-01 14:31 ` KOSAKI Motohiro
2012-05-01 14:37 ` KOSAKI Motohiro
2012-05-01 15:11 ` Jeff Moyer
2012-05-01 15:34 ` KOSAKI Motohiro
2012-05-01 15:38 ` Jeff Moyer
2012-05-01 15:50 ` Nick Piggin
2012-05-01 23:51 ` Andrea Arcangeli
2012-05-02 8:17 ` Jan Kara
2012-05-02 9:09 ` Nick Piggin
2012-05-02 9:18 ` Jan Kara
2012-05-02 19:14 ` KOSAKI Motohiro
2012-05-02 19:23 ` Jan Kara
2012-05-02 19:25 ` KOSAKI Motohiro
2012-05-05 11:28 ` Michael Kerrisk (man-pages)
2012-05-05 15:29 ` KOSAKI Motohiro
2012-05-08 23:10 ` Nick Piggin
2012-05-09 5:35 ` Michael Kerrisk (man-pages)
2012-05-09 7:01 ` Nick Piggin
2012-05-09 7:18 ` Michael Kerrisk (man-pages)
2012-05-10 15:00 ` Jan Kara
2012-05-01 16:15 ` KOSAKI Motohiro
2012-05-01 17:56 ` Michael Kerrisk (man-pages)
2012-05-02 0:34 ` Nick Piggin
2012-05-02 3:04 ` Hugh Dickins
2012-05-02 3:10 ` Nick Piggin
2012-05-02 9:20 ` Jan Kara
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