Re: [PATCH] btrfs-progs: logical-resolve: fix the subvolume path resolution

[Qu Wenruo <wqu@suse.com>]

On 2023/4/19 07:55, David Sterba wrote:
> On Mon, Apr 17, 2023 at 05:48:10PM +0800, Qu Wenruo wrote:
>> [BUG]
>> There is a bug report that "btrfs inspect logical-resolve" can not even
>> handle any file inside non-top-level subvolumes:
>>
>>   # mkfs.btrfs $dev
>>   # mount $dev $mnt
>>   # btrfs subvol create $mnt/subv1
>>   # xfs_io -f -c "pwrite 0 16k" $mnt/subv1/file
>>   # sync
>>   # btrfs inspect logical-resolve 13631488 $mnt
>>   inode 257 subvol subv1 could not be accessed: not mounted
>>
>> This means the command "btrfs inspect logical-resolve" is almost useless
>> for resolving logical bytenr to files.
>>
>> [CAUSE]
>> "btrfs inspect logical-resolve" firstly resolve the logical bytenr to
>> root/ino pairs, then call btrfs_subvolid_resolve() to resolve the path
>> to the subvolume.
>>
>> Then to handle cases where the subvolume is already mounted somewhere
>> else, we call find_mount_fsroot().
>>
>> But if that target subvolume is not yet mounted, we just error out, even
>> if the @path is the top-level subvolume, and we have already know the
>> path to the subvolume.
>>
>> [FIX]
>> Instead of doing unnecessary subvolume mount point check, just require
>> the @path to be the mount point of the top-level subvolume.
> 
> This is a change in the semantics of the command, can't we make it work
> on non-toplevel subvolumes instead? Access to the mounted toplevel
> subvolume is not always provided, e.g. on openSUSE the subvolume layout
> does not mount 5 and there are likely other distros following that
> scheme.

But mounting 5 is not that hard if one is trying to locate the offending 
file.

The original semantics is almost impossible to make real usage, just 
check this mail:

https://lore.kernel.org/linux-btrfs/CAL5DHTFAUCKBmW_j737j8dzRvaBnKWa9Wo5VtvoAgW8f93oR9A@mail.gmail.com/

Doing a mount point search for the subvolume is fine, but requiring each 
subvolume to be mounted while top level subvolume is already mounted is 
not user friendly at all.

Thanks,
Qu