[PATCH] xfs_copy: fix up initial sb buffer read on CRC fs

[Brian Foster]

On Tue, Jul 21, 2015 at 10:11:05AM -0500, Eric Sandeen wrote:
> 
> 
> > On Jul 21, 2015, at 7:31 AM, Brian Foster <bfoster at redhat.com> wrote:
> > 
> >> On Thu, Jul 16, 2015 at 01:20:30PM -0500, Eric Sandeen wrote:
> >> My prior commit, aaf90a2 xfs_copy: fix copy of hard 4k devices
> >> causes xfs_copy to emit a CRC error warning when copying a
> >> CRC filesystem.
> >> 
> >> This is because we are now reading the maximum sector size,
> >> and attempting to verify the CRC based on that (likely incorrect)
> >> length.
> >> 
> >> In xfs_db, we currently just don't verify this read, so it's
> >> not a problem.  In xfs_copy, we almost certainly want to verify.
> >> 
> >> So, first do the maximal read with no verifier; once it's read,
> >> drop that buffer, and re-read with the proper sector size and
> >> verifier.
> >> 
> >> Signed-off-by: Eric Sandeen <sandeen at redhat.com>
> >> ---
> >> 
> >> diff --git a/copy/xfs_copy.c b/copy/xfs_copy.c
> >> index 44a32e8..fd96e15 100644
> >> --- a/copy/xfs_copy.c
> >> +++ b/copy/xfs_copy.c
> >> @@ -654,11 +654,17 @@ main(int argc, char **argv)
> >> 
> >>    memset(&mbuf, 0, sizeof(xfs_mount_t));
> >>    libxfs_buftarg_init(&mbuf, xargs.ddev, xargs.logdev, xargs.rtdev);
> >> +    /* We don't yet know the sector size, so read maximal size */
> >>    sbp = libxfs_readbuf(mbuf.m_ddev_targp, XFS_SB_DADDR,
> >> -                 1 << (XFS_MAX_SECTORSIZE_LOG - BBSHIFT),
> >> -                 0, &xfs_sb_buf_ops);
> >> +                 1 << (XFS_MAX_SECTORSIZE_LOG - BBSHIFT), 0, NULL);
> >>    sb = &mbuf.m_sb;
> >>    libxfs_sb_from_disk(sb, XFS_BUF_TO_SBP(sbp));
> >> +    /* Do it again, now with proper length and verifier */
> >> +    libxfs_putbuf(sbp);
> >> +    libxfs_purgebuf(sbp);
> > 
> > Why the purge? On a quick look, it looks like the buffer cache code
> > would handle this if the buffer size changes.
> > 
> > Hmm, is it to ensure the verification occurs if the buffer size doesn't
> > actually change? If so, I'd suggest to enhance the comment. :)
> > 
> Without the purge, a re-read of a different size at the same offset seems to cause  cache mismatch problems.
> 

Ok, fair enough. Care to update the comment? Otherwise this seems good
to me:

Reviewed-by: Brian Foster <bfoster at redhat.com>

> Eric
> 
> > Brian
> > 
> >> +    sbp = libxfs_readbuf(mbuf.m_ddev_targp, XFS_SB_DADDR,
> >> +                 1 << (sb->sb_sectlog - BBSHIFT),
> >> +                 0, &xfs_sb_buf_ops);
> >> 
> >>    /*
> >>     * For now, V5 superblock filesystems are not supported without -d;
> >> 
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> >