On Fri, Jul 04, 2025 at 07:08:03PM +0930, Qu Wenruo wrote:
> [WEIRD FLAG EXCLUSION]
> Commit ead622674df5 ("btrfs: Do not restrict writes to btrfs devices")
> removes the BLK_OPEN_RESTRICT_WRITES flag when opening the devices
> during mount.
>
> However there is no filesystem except btrfs excluding this flag, which
> is weird, and lacks a proper explanation why we need such an exception.
It's somewhere in the discussions for the patch and there was a
dependency on the series reworking the device opening to be read-only
which I did not consider safe for inclusion. You've worked on that
recently and fixed a few problems and it made it all possible.
The code this patch is removing was added by Linus during merge and was
not obvious that was needed.
> [REASON TO EXCLUDE THAT FLAG]
> Btrfs needs to call btrfs_scan_one_device() to determine the fsid, no
> matter if we're mounting a new fs or an existing one.
>
> But if a fs is already mounted and that BLK_OPEN_RESTRICT_WRITES is
> honored, meaning no other write open is allowed for the block device.
>
> Then we want to mount a subvolume of the mounted fs to another mount
> point, we will call btrfs_scan_one_device() again, but it will fail due
> to the BLK_OPEN_RESTRICT_WRITES flag (no more write open allowed),
> causing only one mount point for the fs.
>
> Thus at that time, we have to exclude the BLK_OPEN_RESTRICT_WRITES to
> allow multiple mount points for one fs.
>
> [WHY IT'S SAFE NOW]
> The root problem is, we do not need to nor should use BLK_OPEN_WRITE for
> btrfs_scan_one_device().
> That function is only to read out the super block, no write at all, and
> BLK_OPEN_WRITE is only going to cause problems for such usage.
>
> The root problem is fixed by patch "btrfs: always open the device
> read-only in btrfs_scan_one_device", so btrfs_scan_one_device() will
> always work no matter if the device is opened with
> BLK_OPEN_RESTRICT_WRITES.
>
> [ENHANCEMENT]
> Just remove the btrfs_open_mode(), as the only call site can be replaced
> with regular sb_open_mode().
>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> Changelog:
> v2:
> - Add more details about the original problem
> And the fix that unintentionally solved the original problem.
Please add it to for-next, thanks.
