Re: [PATCH 2/3] btrfs-progs: ins: logical-resolve: Print message when path cannot be resolved

[Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>]

On 2018/08/04 0:06, David Sterba wrote:
> On Wed, Jul 25, 2018 at 05:20:17PM +0900, Misono Tomohiro wrote:
>> Since BTRFS_IOC_INO_PATHS requires fd of subvolume,
> 
> Does it? AFAICS btrfs_ioctl_ino_to_path gets root that's the containing
> subvolume of the path given by the user.

It is reverse; it returns all paths of given inode number in given subvolume fd.

> 
>> logical-resolve
>> cannot find the path when mount point is not FS_TREE
>> (because the subvolume path cannot be opened).
> 
> Sorry, I don't understand what's the problem here. What you write sounds
> like there's a reproducer. If yes, please post it.

Sure. If mountpoint is FS_TREE, everything is ok:

$ mkfs.btrfs -fq $DEV
$ mount $DEV /mnt

// create snapshot and hardlink
$ btrfs sub create /mnt/sub
$ dd if=/dev/urandom of=/mnt/sub/file bs=1k count=1000
$ btrfs sub snap /mnt/sub /mnt/snap
$ ln /mnt/sub/file /mnt/sub/file2
$ tree --inodes /mnt
/mnt
|-- [    256]  snap
|   `-- [    257]  file
`-- [    256]  sub
    |-- [    257]  file
    `-- [    257]  file2

inode-resolve returns all paths of given inode number in given subvolume:
(it calls IOC_INO_PATHS)
$ btrfs ins inode-resolve 257 /mnt/sub
/mnt/sub/file
/mnt/sub/file2

$ btrfs ins inode-resolve 257 /mnt/snap
/mnt/snap/file

logical-resolve returns all paths containing given logical address in fs:
(it calls IOC_INO_PATHS after IOC_LOGICAL_INO)
$ filefrag -v /mnt/sub/file
Filesystem type is: 9123683e
File size of /mnt/sub/file is 1024000 (250 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..     249:       3392..      3641:    250:             last,shared,eof
/mnt/sub/file: 1 extent found
$ btrfs ins logical-resolve $((3392*4096)) /mnt
/mnt/snap/file
/mnt/sub/file
/mnt/sub/file2


However, when mountpoint is not FS_TREE, above logical-resolve fails:

$ umount /mnt
$ mount -o subvol=sub $DEV /mnt
$ btrfs ins logical-resolve $((3392*4096)) /mnt
ERROR: cannot access '/mnt/snap': No such file or directory

The reasons are
(1) btrfs_list_path_for_root() returns paths from FS_TREE
(2) subvolume path cannot be opened and therefore cannot call IOC_INO_PATHS

(1) may be fixed for mounted subvolume, but if a subvolume is not accessible at all,
there is no way to call IOC_INO_PATHS.
To solve this, we need to pass arbitrary treeid just like INO_LOOKUP ioctl.

Thanks,
Misono