On Mon, Jun 09, 2025 at 11:13:26AM -0700, Darrick J. Wong wrote: > On Fri, Jun 06, 2025 at 03:54:50PM +0200, Miklos Szeredi wrote: > > On Sat, 31 May 2025 at 03:08, Darrick J. Wong <djwong@xxxxxxxxxx> wrote: > > > > > The best reason that I can think of is that normally the process that > > > owns the fd (and hence is releasing it) should be made to wait for > > > the release, because normally we want processes that generate file > > > activity to pay those costs. > > > > That argument seems to apply to all fuse variants. But fuse does get > > away with async release and I don't see why fuseblk would be different > > in this respect. > > > > Trying to hack around the problems of sync release with a task flag > > that servers might or might not have set does not feel a very robust > > solution. > > > > > Also: is it a bug that the kernel only sends FUSE_DESTROY on umount for > > > fuseblk filesystems? I'd have thought that you'd want to make umount > > > block until the fuse server is totally done. OTOH I guess I could see > > > an argument for not waiting for potentially hung servers, etc. > > > > It's a potential DoS. With allow_root we could arguably enable > > FUSE_DESTROY, since the mounter is explicitly acknowledging this DoS > > possibilty. > > <nod> Looking deeper at fuse2fs's op_destroy function, I think most of > the slow functionality (writing group descriptors and the primary super > and fsyncing the device) ought to be done via FUSE_SYNCFS, not > FUSE_DESTROY. If I made that change, I think op_destroy becomes very > fast -- all it does is close the fs and log a message. The VFS unmount > code calls sync_filesystem (which initiates a FUSE_SYNCFS) which sounds > like it would work for fuse2fs. > > Unhappily, libfuse3 doesn't seem to implement it: > > $ git grep FUSE_SYNCFS > doc/libfuse-operations.txt:394:50. FUSE_SYNCFS (50) > include/fuse_kernel.h:186: * - add FUSE_SYNCFS > include/fuse_kernel.h:670: FUSE_SYNCFS = 50, ...and it won't really work anyway since fuse_sync_fs doesn't upcall to the fuse server if sb->s_root == NULL; and we can't do anything at that point anyway because deactivate_locked_super -> fuse_kill_sb_anon has already called fuse_conn_destroy to tear down the connection. --D > > > Thanks, > > Miklos >
