linux-kernel - RE: [PATCH] xfs: Fix possible truncation of log data in xlog_bread

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Date:	Sun, 24 Feb 2013 04:46:30 +0000
From:	Tony Lu <zlu@...era.com>
To:	Dave Chinner <david@...morbit.com>
CC:	Ben Myers <bpm@....com>, "xfs@....sgi.com" <xfs@....sgi.com>,
	Alex Elder <elder@...nel.org>,
	Dave Chinner <dchinner@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: RE: [PATCH] xfs: Fix possible truncation of log data in
 xlog_bread_noalign()

>> For example, if xlog_bread_noalign() wants to read blocks from #1
>> to # 9, in which case the passed parameter blk_no is 1, and nbblks
>> is 8, sectBBsize is 8, after the round down and round up
>> operations, we get blk_no as 0, and nbblks as still 8. We
>> definitely lose the last block of the log data.
>
>Yes, I fully understand that. But I also understand how the log
>works and that this behaviour *should not happen*. That's why
>I'm asking questions about what the problem you are trying to fix.

I am not sure about this, since I saw many reads on non-sector-align blocks even when successfully mounting good XFS partitions. 
-sh-4.1# mount /dev/sda3 /home/
XFS (sda3): Mounting Filesystem
xlog_bread_noalign:blk_no=0,nbblks=1,l_sectBBsize=8
xlog_bread_noalign:blk_no=61447,nbblks=1,l_sectBBsize=8
xlog_bread_noalign:blk_no=0,nbblks=1,l_sectBBsize=8
.......
xlog_bread_noalign:blk_no=8695,nbblks=1,l_sectBBsize=8
xlog_bread_noalign:blk_no=4600,nbblks=4096,l_sectBBsize=8
xlog_bread_noalign:blk_no=8184,nbblks=512,l_sectBBsize=8

And also there is code in xlog_write_log_records() which handles non-sector-align reads and writes.

	/* We may need to do a read at the start to fill in part of
	 * the buffer in the starting sector not covered by the first
	 * write below.
	 */
	balign = round_down(start_block, sectbb);
	if (balign != start_block) {
		error = xlog_bread_noalign(log, start_block, 1, bp);
		if (error)
			goto out_put_bp;

		j = start_block - balign;
	}

>Ramdisks don't persist over a reboot, so you must have had some
>other way of reproducing the problem. Can you tell me how you
>reproduced it on a ramdisk? Better yet, send me a script that
>reproduces the problem?

I will try to reproduce it. Basically it is a loop of mount, creating many files and unmount.

Thanks
-Tony
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