Incorrect error message from erofs "backed by file" in 6.12-rc

[Gao Xiang]

Hi Allison,

(try to +Cc Christian)

On 2024/10/2 20:58, Allison Karlitskaya wrote:
> hi,
> 
> In context of my work on composefs/bootc I've been testing the new support for directly mounting files with erofs (ie: without a loopback device) and it's working well.  Thanks for adding this feature --- it's a huge quality of life improvement for us.
> 
> I've observed a strange behaviour, though: when mounting a file as an erofs, if you read() the filesystem context fd, you always get the following error message reported: Can't lookup blockdev.
> 
> That's caused by the code in erofs_fc_get_tree() trying to call get_tree_bdev() and recovering from the error in case it was ENOTBLK and CONFIG_EROFS_FS_BACKED_BY_FILE.  Unfortunately, get_tree_bdev() logs the error directly on the fs_context, so you get the error message even on successful mounts.
> > It looks something like this at the syscall level:
> 
> fsopen("erofs", FSOPEN_CLOEXEC)         = 3
> fsconfig(3, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
> fsconfig(3, FSCONFIG_SET_STRING, "source", "/home/lis/src/mountcfs/cfs", 0) = 0
> fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
> fsmount(3, FSMOUNT_CLOEXEC, 0)          = 5
> move_mount(5, "", AT_FDCWD, "/tmp/composefs.upper.KuT5aV", MOVE_MOUNT_F_EMPTY_PATH) = 0
> read(3, "e /home/lis/src/mountcfs/cfs: Can't lookup blockdev\n", 1024) = 52
> 
> This is kernel 6.12.0-0.rc0.20240926git11a299a7933e.13.fc42.x86_64 from Fedora Rawhide.
> 
> It's a pretty minor issue, but it sent me on a wild goose chase for an hour or two, so probably it should get fixed before the final release.
> 

Sorry for late response. I'm on vacation recently.

Yes, I also observed this message, but I'm not sure
how to handle it better.  Indeed, the message itself
is out of get_tree_bdev() as you said.

Yet I tend to avoid unnecessary extra lookup_bdev()
likewise to confirm the type of the source in advance,
since the majority mount type of EROFS is still
bdev-based instead file-based so I tend to make
file-based mount as a fallback...

Hi Christian, if possible, could you give some other
idea to handle this case better? Many thanks!

Thanks,
Gao Xiang

> Thanks again for this awesome feature!
> 
> Allison Karlitskaya