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Re: [PATCH 1/9] xfs: skip verification on initial "guess" superblock rea

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: Re: [PATCH 1/9] xfs: skip verification on initial "guess" superblock read
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 19 Feb 2014 14:36:31 +1100
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1392767549-25574-2-git-send-email-sandeen@xxxxxxxxxx>
References: <1392767549-25574-1-git-send-email-sandeen@xxxxxxxxxx> <1392767549-25574-2-git-send-email-sandeen@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Feb 18, 2014 at 05:52:21PM -0600, Eric Sandeen wrote:
> When xfs_readsb() does the very first read of the superblock,
> it makes a guess at the length of the buffer, based on the
> sector size of the underlying storage.  This may or may
> not match the filesystem sector size in sb_sectsize, so
> we can't i.e. do a CRC check on it; it might be too short.
> 
> In fact, mounting a filesystem with sb_sectsize larger
> than the device sector size will cause a mount failure
> if CRCs are enabled, because we are checksumming a length
> which exceeds the buffer passed to it.
> 
> So always read twice; the first time we read with NULL
> buffer ops to skip verification; then set the proper
> read length, hook up the proper verifier, and give it
> another go.
> 
> Once we are sure that we've got the right buffer length,
> we can also use bp->b_length in the xfs_sb_read_verify,
> rather than the less-trusted on-disk sectorsize for
> secondary superblocks.  Before this we ran the risk of
> passing junk to the crc32c routines, which didn't always
> handle extreme values.
> 
> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>

Looks good.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>


-- 
Dave Chinner
david@xxxxxxxxxxxxx

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