Re: [PATCH v5] erofs: accept source file descriptor via fsconfig

[Giuseppe Scrivano <gscrivan@redhat.com>]

Gao Xiang <hsiangkao@gmx.com> writes:

> On Mon, Aug 03, 2026 at 02:01:41PM +0200, Giuseppe Scrivano wrote:
>> Chao Yu <chao@kernel.org> writes:
>> 
>> > On 7/29/26 00:05, 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 <gscrivan@redhat.com>
>> >> ---
>> >> v4: https://lore.kernel.org/linux-fsdevel/20260717134147.1602735-1-gscrivan@redhat.com/
>> >> v3: https://lore.kernel.org/linux-fsdevel/20260714154917.489993-1-gscrivan@redhat.com/
>> >> v2: https://lore.kernel.org/linux-fsdevel/20260711071137.4130824-1-gscrivan@redhat.com/
>> >> 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.
>> >
>> > Do we need to add an entry to describe the new mount option source= in
>> > "Mount options" section in erofs.rst?
>> >
>> > Thanks,
>> 
>> I've another patch "erofs: reuse superblock for file-backed mounts" that
>> also touches erofs.rst and it is currently based on top of this version.
>> I wonder what is your preferred way to handle them.
>
> EROFS dev branch usually has a small number of patches. Since Chao gave
> a new suggestion, I think a v6 is needed (but you could add all
> previous reviewed-by:).
>
> And "erofs: reuse superblock for file-backed mounts" needs to be rebased
> too, I will find time to play with it this week (or you could add a
> testcase in erofs-utils experimental-tests if you have time.)
>
>> 
>> Will maintainers deal with conflicts or do I need to submit them as a
>> single series?
>> 
>> Do you prefer a new submission v6 with the following fixup?
>> 
>> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
>> index c972a869f3e9..49c4e7dbc5d6 100644
>> --- a/Documentation/filesystems/erofs.rst
>> +++ b/Documentation/filesystems/erofs.rst
>> @@ -137,6 +137,8 @@ fsoffset=%llu          Specify block-aligned filesystem offset for the primary d
>>  inode_share            Enable inode page sharing for this filesystem.  Inodes with
>>                         identical content within the same domain ID can share the
>>                         page cache.
>> +source=%s              (For file-backed mounts) Specify the backing image as a path
>> +                       or as an already-opened file descriptor.
>
> Not quite sure "an already-opened file descriptor" is "%s", but I'm fine
> since it's quite a minor one (it's unneeded to nitpick the
> documentation).

thanks, I've sent v6 as well as the rebase of "erofs: reuse superblock
for file-backed mounts" on top of it.

Regards,
Giuseppe