On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote: > /proc/$PID/mountinfo contains a field for the device number of the > filesystem at each mount. > > This is taken from the superblock ->s_dev field, which is correct for > every filesystem except btrfs. A btrfs filesystem can contain multiple > subvols which each have a different device number. If (a directory > within) one of these subvols is mounted, the device number reported in > mountinfo will be different from the device number reported by stat(). > > This confuses some libraries and tools such as, historically, findmnt. > Current findmnt seems to cope with the strangeness. > > So instead of using ->s_dev, call vfs_getattr_nosec() and use the ->dev > provided. As there is no STATX flag to ask for the device number, we > pass a request mask for zero, and also ask the filesystem to avoid > syncing with any remote service. Hard NAK. You are putting IO (potentially - network IO, with no upper limit on the completion time) under namespace_sem. This is an instant DoS - have a hung NFS mount anywhere in the system, try to cat /proc/self/mountinfo and watch a system-wide rwsem held shared. >From that point on any attempt to take it exclusive will hang *AND* after that all attempts to take it shared will do the same. Please, fix BTRFS shite in BTRFS. Without turning a moderately unpleasant problem (say, unplugged hub on the way to NFS server) into something that escalates into buggered clients. Note that you have taken out any possibility to e.g. umount -l /path/to/stuck/mount, along with any chance of clear shutdown of the client. Not going to happen. NAKed-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
