Re: [PATCH 1/2] repair: fix unnecessary secondary scan if only last sb is corrupt

[Brian Foster <bfoster@redhat.com>]

On Thu, Jan 15, 2015 at 03:44:43PM -0600, Eric Sandeen wrote:
> On 1/13/15 2:08 PM, Brian Foster wrote:
> > verify_set_primary_sb() scans the secondary superbocks based on the
> > geometry specified in the primary and determines the most likely correct
> > geometry by tracking how many superblocks are consistent across the set.
> > The most frequent geometry is copied into the primary superblock. The
> > return value is checked by the caller (phase1()) to determine whether a
> > brute force secondary scan is necessary.
> > 
> > This generally occurs when not enough secondary sb's are consistent to
> > declare the geometry correct. If enough secondaries are consistent,
> > verify_set_primary_sb() returns the status of the last secondary sb that
> > was scanned. Corruptions to secondary supers other than the last are
> > thus resolved fine. If the last secondary is corrupt, however, an error
> > is returned to phase1(). This causes a brute force scan even if enough
> > supers were found to repair the last secondary.
> > 
> > Move the initialization of retval to after the sb scan to return an
> > error only if not enough secondary supers were found to declare a
> > correct geometry.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> Nice.  Brute-force scan is awful, doing it when unnecessary stinks!  :)
> 
> could this be fstest-ed?
> 

Yeah, the problem is easy to reproduce with xfs_db (just corrupt the
last sb). I think a test that runs repair through a few sets of sb
corruptions should be easy enough. I'll look into it.

Another situation I ran into while playing with these is the brute force
scan finding an older secondary (i.e., from a previous mkfs) and
eventually falling over based on its geometry rather than continuing to
scan for the a secondary of the current fs. I wasn't sure what was going
on at the time so I zeroed off the bdev and retested to confirm that was
the problem. I wonder how likely something like that might be in the
real world...

> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
> 

Thanks for reviewing these!

Brian

> > ---
> >  repair/sb.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/repair/sb.c b/repair/sb.c
> > index ad27756..dc154f7 100644
> > --- a/repair/sb.c
> > +++ b/repair/sb.c
> > @@ -724,7 +724,6 @@ verify_set_primary_sb(xfs_sb_t		*rsb,
> >  	 * sector size rather than the sector size in @rsb.
> >  	 */
> >  	size = NUM_AGH_SECTS * (1 << (XFS_MAX_SECTORSIZE_LOG));
> > -	retval = 0;
> >  	list = NULL;
> >  	num_ok = 0;
> >  	*sb_modified = 0;
> > @@ -779,6 +778,7 @@ verify_set_primary_sb(xfs_sb_t		*rsb,
> >  	/*
> >  	 * see if we have enough superblocks to bother with
> >  	 */
> > +	retval = 0;
> >  	if (num_ok < num_sbs / 2) {
> >  		retval = XR_INSUFF_SEC_SB;
> >  		goto out_free_list;
> > @@ -868,5 +868,5 @@ out_free_list:
> >  	free_geo(list);
> >  	free(sb);
> >  	free(checked);
> > -	return(retval);
> > +	return retval;
> >  }
> > 
> 
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