Hi Giuseppe,
On 2026/7/27 16:01, Giuseppe Scrivano wrote:
Gao Xiang <[email protected]> writes:
Hi Christian,
On 2026/7/23 22:46, Christian Brauner wrote:
I'm not quite sure if I catched the point, I think Giuseppe's patch here
tried to record `file` into `sbi->dif0.file` (which indicates the primary
"device" later.)
And if `sbi->dif0.file` is set up by erofs_fc_parse_source(),
erofs_fc_get_tree() will just use `sbi->dif0.file` instead of
`fc->source` according to this patch.
Oh, so you only do it for file-backed mounts. Do you only allow
regular
files or do you also support block devices with
CONFIG_EROFS_FS_BACKED_BY_FILE?
Block devices with CONFIG_EROFS_FS_BACKED_BY_FILE are supported,
but with only `fc->source` (not this way.)
That is the limitation I see in Giuseppe's patch. I'd hoped
bdev-backed mounts could work the same way, but that would require
changes to the VFS flow.
Since this is a side improvement, I think it's fine as long as
it's documented somewhere, and I do hope Giuseppe can at least
address the documentation.
would something like the following be enough?
diff --git a/Documentation/filesystems/erofs.rst
b/Documentation/filesystems/erofs.rst
index 4230884fb359..768e1d43dfcc 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -139,6 +139,29 @@ 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 can mount filesystem
+images stored as regular files directly, without requiring a loopback block
+device. The source can be specified either by path or by passing an
+already-opened file descriptor via ``fsconfig(fd, FSCONFIG_SET_FD, "source",
+NULL, source_fd)``. Only regular files are accepted; block devices must use
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 via
``fsconfig(fd, FSCONFIG_SET_FD, "source", NULL, source_fd)``.
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.
It's just my own sketch of this; you could just fold this into this patch
with modification (I'm not quite good at English.)
Also it lacks how `fc->source is filled` when source_fd is specified, we
may need to document here as well (and hopefully vfs maintainers can ack
on this so it can be stable.)
+the standard block device mount path.
+
+The backing file content must remain stable for the lifetime of the mount.
+EROFS never writes to it, but concurrent modifications by other processes lead
+to undefined behavior.
Yes, I explained to Christian but I don't think it should be included
in this patch, maybe we need to document this as a new section
in a seperate patch later (possibly as a formal security model.)
+
+Ioctls
+======
+
+``EROFS_IOC_GET_SOURCE_FD``
+ Return a read-only file descriptor (``O_CLOEXEC``) for the backing file of a
+ file-backed mount. Returns ``-ENOENT`` on block-device-backed mounts.
+ Requires ``CAP_SYS_ADMIN`` in the initial user namespace (returns ``-EPERM``
+ otherwise).
I hope document this part in the corresponding patch but I guess we have
to get a consensus between filesystems first.
Thanks,
Gao Xiang
