Re: [PATCH 1/2] xfs: correct the sb_rgcount when the disk not support rt volume

[Long Li <leo.lilong@huawei.com>]

On Mon, Jan 06, 2025 at 11:52:20AM -0800, Darrick J. Wong wrote:
> On Tue, Dec 31, 2024 at 10:34:22AM +0800, Long Li wrote:
> > When mounting an xfs disk that incompat with metadir and has no realtime
> > subvolume, if CONFIG_XFS_RT is not enabled in the kernel, the mount will
> > fail. During superblock log recovery, since mp->m_sb.sb_rgcount is greater
> > than 0, updating the last rtag in-core is required, however, without
> > CONFIG_XFS_RT enabled, xfs_update_last_rtgroup_size() always returns
> > -EOPNOTSUPP, leading to mount failure.
> 
> Didn't we fix the xfs_update_last_rtgroup_size stub to return 0?
> 
> --D

Indeed, when CONFIG_XFS_RT is not enabled, xfs_update_last_rtgroup_size() should
return 0, as returning an error is meaningless.

1) For kernels without CONFIG_XFS_RT, mounting an image with realtime subvolume will
fail at xfs_rtmount_init().

2) For kernels without CONFIG_XFS_RT, mounting an image without realtime subvolume
should succeed.

However, in the current scenario, should sb_rgcount be initialized to 0 ? it will 
consistent with metadir feature is enabled. The xfs-documentation [1] describes 
sb_rgcount as follows:

"Count of realtime groups in the filesystem, if the XFS_SB_FEAT_RO_INCOMPAT_METADIR
feature is enabled. If no realtime subvolume exists, this value will be zero."

[1] https://git.kernel.org/pub/scm/fs/xfs/xfs-documentation.git/tree/design/XFS_Filesystem_Structure/superblock.asciidoc

Thanks,
Long Li

> 
> > Initializing sb_rgcount as 1 is incorrect in this scenario. If no
> > realtime subvolume exists, the value of sb_rgcount should be set
> > to zero. Fix it by initializing sb_rgcount based on the actual number
> > of realtime blocks.
> > 
> > Fixes: 87fe4c34a383 ("xfs: create incore realtime group structures")
> > Signed-off-by: Long Li <leo.lilong@huawei.com>
> > ---
> >  fs/xfs/libxfs/xfs_sb.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> > index 3b5623611eba..1ea28f04b75a 100644
> > --- a/fs/xfs/libxfs/xfs_sb.c
> > +++ b/fs/xfs/libxfs/xfs_sb.c
> > @@ -830,7 +830,7 @@ __xfs_sb_from_disk(
> >  		to->sb_rsumino = NULLFSINO;
> >  	} else {
> >  		to->sb_metadirino = NULLFSINO;
> > -		to->sb_rgcount = 1;
> > +		to->sb_rgcount = to->sb_rblocks > 0 ? 1 : 0;
> >  		to->sb_rgextents = 0;
> >  	}
> >  }
> > -- 
> > 2.39.2
> > 
> > 
>