Re: [PATCH] Describe race of direct read and fork for unaligned buffers
From: KOSAKI Motohiro <hidden>
Date: 2012-05-01 14:32:07
Also in:
linux-man, lkml
On Mon, Apr 30, 2012 at 5:30 AM, Jan Kara [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted> 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
Hello, Thank you revisit this. But as far as my remember is correct, this issue is NOT unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e. DIRECT_IO w/ multi thread process should not use fork(). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>