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

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jan 07, 2025 at 09:11:23PM +0800, Long Li wrote:
> 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

Ah, I see your point finally -- if there's no realtime section, then
there's no need to allocate any incore rtgroups, nor is there any point
to set sb_rgcount==1.

That said, I think the correct tags here are:
Cc: <stable@vger.kernel.org> # v6.13-rc1
Fixes: 96768e91511bfc ("xfs: define the format of rt groups")

because 96768e91511bfc is the commit that actually added "to->sb_rgcount
= 1;".

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D


> 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
> > > 
> > > 
> > 
>