Combine inode creation with opening a file.

There are six separate objects that are being set up: the backing inode,
dentry and file, and the overlay inode, dentry and file.  Cleanup in case
of an error is a bit of a challenge and is difficult to test, so careful
review is needed.

All tmpfile testcases except generic/509 now run/pass, and no regressions
are observed with full xfstests.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>

commit 0a31bd5f

 ("KMEM_CACHE(): simplify slab cache creation")
introduces a new macro.
Use the new KMEM_CACHE() macro instead of direct kmem_cache_create
to simplify the creation of SLAB caches.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240131070941.135178-1-chentao@kylinos.cn


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Assert that the file object is allocated in a backing_file container
so that file_user_path() could be used to display the user path and
not the backing file's path in /proc/<pid>/maps.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

There is not much in those helpers, but it makes sense to have them
logically next to the backing_file_{read,write}_iter() helpers as they
may grow more common logic in the future.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Overlayfs submits files io to backing files on other filesystems.
Factor out some common helpers to perform io to backing files, into
fs/backing-file.c.

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Link: https://lore.kernel.org/r/CAJfpeguhmZbjP3JLqtUy0AdWaHOkAPWeP827BBWwRFEAUgnUcQ@mail.gmail.com


Signed-off-by: Amir Goldstein <amir73il@gmail.com>

In preparation for factoring out some backing file io helpers from
overlayfs, move backing_file_open() into a new file fs/backing-file.c
and header.

Add a MAINTAINERS entry for stackable filesystems and add a Kconfig
FS_STACK which stackable filesystems need to select.

For now, the backing_file struct, the backing_file alloc/free functions
and the backing_file_real_path() accessor remain internal to file_table.c.
We may change that in the future.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>