[PATCH v2 1/2] anon_inodes: allow external inode allocations

[Al Viro]

On Wed, Oct 02, 2013 at 01:24:25PM +0200, David Herrmann wrote:
> DRM core shares a single address_space across all inodes that belong to
> the same DRM device. This allows efficient unmapping of user-space
> mappings during buffer eviction. However, there is no easy way to get a
> shared address_space for DRM devices during initialization. Therefore, we
> currently delay this step until the first ->open() and save the given
> inode for future use.
> 
> This causes ugly delayed initialization throughout the DRM core. TTM
> devices end up without a dev_mapping pointer and we have to carefully
> respect any underlying filesystem implementation so we don't corrupt the
> inode->i_mapping and inode->i_data fields.
> 
> We can avoid this if we were allowed to allocate an anonymous inode for
> each DRM device. We only have to set file->f_mapping during ->open()
> and no longer need to adjust inode mappings. As fs/anon_inodes.c already
> provides a minimal internal FS mount, we extend it to also provide
> anonymous inodes for use in drivers like DRM.
> 
> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
> Wanted-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Told-to-stop-abusing-anon_inodes-by: Al Viro <viro at zeniv.linux.org.uk>

Note that anon_inode_getfile_private() is not going to survive - for exact
same reasons.  It's already removed in vfs.git#experimental, shortly in
#for-next as well.  Do NOT extend fs/anon_inodes.c; if you need a minimal
internal fs mount, just do what fs/aio.c does in the same branch.