Re: [PATCH] xfs: do not unconditionally enable hasalign feature on V5 filesystems

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Wednesday, February 15, 2017 11:03:11 AM Eric Sandeen wrote:
> On 2/15/17 10:13 AM, Eric Sandeen wrote:
> >> The root cause of the problem is due to the fact that
> >> xfs_sb_version_hasalign() returns true when we are working on a V5
> >> filesystem. Due to this args.minalignslop (in xfs_ialloc_ag_alloc())
> >> gets the unsigned equivalent of -1 assigned to it. This later causes
> >> alloc_len in xfs_alloc_space_available() to have a value of 0. In such a
> >> scenario when args.total is also 0, the assert statement
> >> "ASSERT(args->maxlen > 0);" fails.
> > Hm, the intent of the _haslign() function is to say that V5 must always
> > imply the "alignbit" - i.e. we don't want to grow an infinite feature
> > matrix, and by the time you get to V5 supers, there are many things which
> > cannot be turned on or off, such as this feature.
> > 
> > So what happens here... xfs_ialloc_ag_alloc does:
> > 
> > args.minalignslop = xfs_ialloc_cluster_alignment(args.mp) - 1;
> > 
> > so you're saying that cluster_alignment comes out as 0?
> > 
> > That function is checking _hasalign:
> > 
> > static inline int
> > xfs_ialloc_cluster_alignment(
> >         struct xfs_mount        *mp)
> > {
> >         if (xfs_sb_version_hasalign(&mp->m_sb) &&
> >             mp->m_sb.sb_inoalignmt >=
> >                         XFS_B_TO_FSBT(mp, mp->m_inode_cluster_size))
> >                 return mp->m_sb.sb_inoalignmt;
> >         return 1;
> > }
> > 
> > So are you saying that this function returns 0?  That would imply that
> > sb_inoalignmt and m_inode_cluster_size are both zero, yes?  Is this
> > what you see?
> 
> Sorry, I guess that means XFS_B_TO_FSBT(mp, mp->m_inode_cluster_size)) is
> zero; inode cluster size is 8192 in this case I think, and that is in fact
> 0 filesystem blocks when computed with this macro.

Yes, sb_inoalignmt is indeed 0 because of the following code from
main() in xfsprogs/mkfs/xfs_mkfs.c,

	if (sb_feat.inode_align) {
		int	cluster_size = XFS_INODE_BIG_CLUSTER_SIZE;
		if (sb_feat.crcs_enabled)
			cluster_size *= isize / XFS_DINODE_MIN_SIZE;
		sbp->sb_inoalignmt = cluster_size >> blocklog;
		sb_feat.inode_align = sbp->sb_inoalignmt != 0;
	}

And XFS_B_TO_FSBT(mp, mp->m_inode_cluster_size)) returns 0 as well. Hence the
condition (xfs_sb_version_hasalign(&mp->m_sb) && mp->m_sb.sb_inoalignmt >=
XFS_B_TO_FSBT(mp, mp->m_inode_cluster_size)) evaluates to true.

> 
> I need to think about this a little bit to convince myself that the inode
> alignment bit really /should/ be off for a filesystem of this geometry, vs
> changing the macro to recognize the case.
> 
> -Eric
> 

-- 
chandan