On Mon, Dec 11, 2017 at 8:41 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > On Mon, Dec 11, 2017 at 8:04 AM, NeilBrown <neilb@xxxxxxxx> wrote: >> If a filesystem does not set sb->s_export_op, then it >> does not support filehandles and export_fs_encode_fh() >> and exportfs_encode_inode_fh() should not be called. >> They will use export_encode_fh() is which is a default >> that uses inode number generation number, but in general >> they may not be stable. >> >> So change exportfs_encode_inode_fh() to return FILEID_INVALID >> if called on an unsupported Filesystem. Currently only >> notify/fdinfo can do that. >> > > I wish you would leave this check to the caller, maybe add a helper > exportfs_can_decode_fh() for callers to use. > > Although there are no current uses for it in-tree, there is value in > being able to encode a unique file handle even when it cannot be > decoded back to an open file. > > I am using this property in my fanotify super block watch patches, > where the object identifier on the event is an encoded file handle > of the object, which delegates tracking filesystem objects to > userspace and prevents fanotify from keeping elevated refcounts > on inodes and dentries. > > There are quite a few userspace tools out there that are checking > that st_ino hasn't changed on a file between non atomic operations. > Those tools (or others) could benefit from a unique file handle if > we ever decide to provide a relaxed version of name_to_handle_at(). > And this change need a clause about not breaking userspace. Pavel, Will this break any version of criu in the wild? Amir.
