Hi Jan, On Wed, Jul 29, 2026 at 11:57:25AM +0200, Jan Kara wrote: > On Wed 29-07-26 09:40:50, Gao Xiang wrote: > > On Tue, Jul 28, 2026 at 06:05:32PM +0200, Giuseppe Scrivano wrote: > > > Allow userspace to pass an already-opened file descriptor as the mount > > > source instead of a path string. This is useful for tools that already > > > hold an fd to the image, such as composefs reusing an existing erofs > > > backing file. > > > > > > Signed-off-by: Giuseppe Scrivano <[email protected]> > > > --- > > > v4: > > > https://lore.kernel.org/linux-fsdevel/[email protected]/ > > > v3: > > > https://lore.kernel.org/linux-fsdevel/[email protected]/ > > > v2: > > > https://lore.kernel.org/linux-fsdevel/[email protected]/ > > > v1: https://lore.kernel.org/linux-fsdevel/ak5GfvVfWLJU1EwK@debian/ > > > > > > Documentation/filesystems/erofs.rst | 15 ++++++ > > > fs/erofs/super.c | 73 ++++++++++++++++++++++++----- > > > 2 files changed, 77 insertions(+), 11 deletions(-) > > > > > > diff --git a/Documentation/filesystems/erofs.rst > > > b/Documentation/filesystems/erofs.rst > > > index 4230884fb359..774e8b236d09 100644 > > > --- a/Documentation/filesystems/erofs.rst > > > +++ b/Documentation/filesystems/erofs.rst > > > @@ -139,6 +139,21 @@ inode_share Enable inode page sharing for > > > this filesystem. Inodes wi > > > page cache. > > > =================== > > > ========================================================= > > > > > > +File-backed mounts > > > +================== > > > + > > > +When CONFIG_EROFS_FS_BACKED_BY_FILE is enabled, EROFS file-backed images > > > +can be mounted directly without a loopback block device. The backing > > > file > > > +can be given either as a path, or as an already-opened file descriptor. > > > + > > > +When a file descriptor is used, the kernel resolves its path and records > > > it > > > +so that /proc/mounts and similar interfaces can still report the mount > > > +source. > > > + > > > +Only regular files are accepted as backing files; to mount an image that > > > +resides on a block device, use the traditional block device mount path > > > +instead. > > > > Reviewed-by: Gao Xiang <[email protected]> > > > > Hi Christian, Jan, could you ack this (`fc->source` filling when > > `source_fd` is used) if possible? > > Whenever I see a use of d_path() (or file_path() in this case) I'm a bit > watchful because its result is inherently racy and unsafe. But in this case > the result is only used for alloc_vfsmnt() which uses it for mnt_devname so > in the worst case we'll get an unreliable path in /proc/mounts or similar. > I guess that's acceptable. So feel free to add:
Thanks for your reply and comment. Yeah, the result of d_path() can become outdated later since it only uses seqcounts to ensure that within the process, but here I also think "mnt_devname" usage doesn't really matter (but I hope filling like this is fine and stable in the long term.) > > Acked-by: Jan Kara <[email protected]> Thanks! Thanks, Gao Xiang > > Honza > -- > Jan Kara <[email protected]> > SUSE Labs, CR >
