Re: [PATCH 1/2] ext4, dax: update dax check to skip journal inode

[Kani, Toshi]

On Wed, 2018-09-12 at 18:20 +0200, Jan Kara wrote:
> On Wed 12-09-18 15:47:12, Kani, Toshi wrote:
> > On Wed, 2018-09-12 at 11:24 +0200, Jan Kara wrote:
> > > On Tue 11-09-18 09:42:45, Toshi Kani wrote:
> > > > Ext4 mount path calls ext4_iget() to obtain the journal inode.  This
> > > > inode does not support DAX, and 'ext4_da_aops' needs to be set.  It
> > > > currently works for the DAX mount case because ext4_iget() always set
> > > > 'ext4_da_aops' to any regular files.
> > > > 
> > > >   ext4_fill_super
> > > >     ext4_load_journal
> > > >       ext4_get_journal_inode
> > > >         ext4_iget
> > > > 
> > > > In preparation to fix ext4_iget() to set 'ext4_dax_aops' for DAX files,
> > > > update ext4_should_use_dax() to return false for the journal inode.
> > > > 
> > > > Fixes: 5f0663bb4a64f588f0a2dd6d1be68d40f9af0086
> > > > Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> > > > Cc: Jan Kara <jack@xxxxxxx>
> > > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > > Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> > > > Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
> > > 
> > > The journal inode is similar to any other 'system' inode we have in ext4.
> > > We don't really expect any of the address space operations to be called for
> > > it because we don't use page cache with these inodes. Very similar
> > > situation is with e.g. quota files. So to me it seems this patch is not
> > > really necessary. Or did you observe any bad effects without this patch?
> > 
> > Yes, without this change, mount fails with the error below.  I believe
> > .bmap operation in ext4_da_aops is necessary for this case.
> > 
> >   jbd2_journal_init_inode: Cannot locate journal superblock
> >   EXT4-fs
> > (pmem1): Could not load journal inode
> 
> Ah, OK, forgot about this. Then how about adding:
> 	.bmap                   = ext4_bmap,
> 
> to ext4_dax_aops? .bmap is completely fine for DAX inodes and there's no
> reason to disallow it.

Yes, mount -o dax succeeds with the added ext4_bmap in ext4_dax_aops.

Thanks!
-Toshi