Re: [PATCH] btrfs: always scan a single device when mounted

On 2/5/24 23:13, David Sterba wrote:

There are reports that since version 6.7 update-grub fails to find the device of the root on systems without initrd and on a single device.

This looks like the device name changed in the output of /proc/self/mountinfo:

6.5-rc5 working

18 1 0:16 / / rw,noatime - btrfs /dev/sda8 ...

6.7 not working:

17 1 0:15 / / rw,noatime - btrfs /dev/root ...

and "update-grub" shows this error:

/usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?)

This looks like it's related to the device name, but grub-probe recognizes the "/dev/root" path and tries to find the underlying device. However there's a special case for some filesystems, for btrfs in particular.

The generic root device detection heuristic is not done and it all relies on reading the device infos by a btrfs specific ioctl. This ioctl returns the device name as it was saved at the time of device scan (in this case it's /dev/root).

The change in 6.7 for temp_fsid to allow several single device filesystem to exist with the same fsid (and transparently generate a new UUID at mount time) was to skip caching/registering such devices.

This also skipped mounted device.

It's reasonable. However is there any logs from the debug message to confirm?

One step of scanning is to check if the device name hasn't changed, and if yes then update the cached value.

This broke the grub-probe as it always read the device /dev/root and couldn't find it in the system. A temporary workaround is to create a symlink but this does not survive reboot.

The right fix is to allow updating the device path of a mounted filesystem even if this is a single device one.

And at the same time, if the device is not mounted, it still does not register the single device. As in the original design.

This does not affect the temp_fsid feature, the UUID of the mounted filesystem remains the same and the matching is based on device major:minor which is unique per mounted filesystem.

It looks like the logic in find_fsid_by_device() does not verify if the %fsid_fs_devices and %devt_fs_devices are the same. I am concerned that if they aren't, and %devt_fs_devices is matched by devt only, it implies that the rest of the populated values in fs_devices may be inconsistent (from the super).

I had a bunch of test cases for temp-fsid, I need to (find them) and send it out.

As the main part of device scanning and list update is done in device_list_add() that handles all corner cases and locking, it is extended to take a parameter that tells it to do everything as before, except adding a new device entry.

Hm. %new_device_added was for btrfs_scan_one_device() so that it can call btrfs_free_stale_devices(), removing any stale devices if present. However, theoretically, there shouldn't be any stale devices. Let's keep it for now, as the find_fsid_by_device() logic might return incorrect fs_devices (I need to confirm this again).

This covers the path when the device (that exists for all mounted devices) name changes, updating /dev/root to /dev/sdx. Any other single device with filesystem is skipped.

Note that if a system is booted and initial mount is done on the /dev/root device, this will be the cached name of the device. Only after the command "btrfs device rescan" it will change as it triggers the

"btrfs device scan"

rename.

The fix was verified by users whose systems were affected.

CC: stable@vger.kernel.org # 6.7+ Fixes: bc27d6f0aa0e ("btrfs: scan but don't register device on single device filesystem") Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=218353 Link: https://lore.kernel.org/lkml/CAKLYgeJ1tUuqLcsquwuFqjDXPSJpEiokrWK2gisPKDZLs8... Signed-off-by: David Sterba dsterba@suse.com

---

fs/btrfs/volumes.c | 30 ++++++++++++++---------------- 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 474ab7ed65ea..f2c2f7ca5c3d 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -738,6 +738,7 @@ static noinline struct btrfs_device *device_list_add(const char *path, bool same_fsid_diff_dev = false; bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) & BTRFS_FEATURE_INCOMPAT_METADATA_UUID);

• bool can_create_new = *new_device_added;

if (btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_CHANGING_FSID_V2) { btrfs_err(NULL, @@ -753,6 +754,7 @@ static noinline struct btrfs_device *device_list_add(const char *path, return ERR_PTR(error); }

• *new_device_added = false;

fs_devices = find_fsid_by_device(disk_super, path_devt, &same_fsid_diff_dev); if (!fs_devices) { @@ -804,6 +806,15 @@ static noinline struct btrfs_device *device_list_add(const char *path, return ERR_PTR(-EBUSY); }

• if (!can_create_new) {
  

• 	pr_info(
  

• "BTRFS: device fsid %pU devid %llu transid %llu %s skip registration scanned by %s (%d)\n",

• 		disk_super->fsid, devid, found_transid, path,
  

• 		current->comm, task_pid_nr(current));
  

• 	mutex_unlock(&fs_devices->device_list_mutex);
  

• 	return NULL;
  

• }
  

I just ran the temp-fsid test cases attached here. Comparing the output with and without this patch seems to be breaking something in the temp-fsid feature. I need to further verify. I will convert this test to fstests. However, in the meantime, it can be used to verify.

The testcases need to 'run' command from git@github.com:asj/run.git. sb command is also attached here.

To run-the-test case: save attahced 'sb' and 'run' command from github to a BIN directory. update the devices in the file 't' and run it. ./t (with and without patch)

Sorry for the messy self use testscripts for now. I am converting them to fstests.

Thanks, Anand

nofs_flag = memalloc_nofs_save();
device = btrfs_alloc_device(NULL, &devid,
			    disk_super->dev_item.uuid, path);

@@ -1355,27 +1366,14 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, goto error_bdev_put; }

• if (!mount_arg_dev && btrfs_super_num_devices(disk_super) == 1 &&

•    !(btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_SEEDING)) {
  

• dev_t devt;
  

• ret = lookup_bdev(path, &devt);
  

• if (ret)
  

• 	btrfs_warn(NULL, "lookup bdev failed for path %s: %d",
  

• 		   path, ret);
  

• else
  

• 	btrfs_free_stale_devices(devt, NULL);
  

• pr_debug("BTRFS: skip registering single non-seed device %s\n", path);
  

• device = NULL;
  

• goto free_disk_super;
  

• }

• if (mount_arg_dev || btrfs_super_num_devices(disk_super) != 1 ||

•    (btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_SEEDING))
  

• new_device_added = true;
  

device = device_list_add(path, disk_super, &new_device_added); if (!IS_ERR(device) && new_device_added) btrfs_free_stale_devices(device->devt, device); -free_disk_super: btrfs_release_disk_super(disk_super); error_bdev_put: