On Fri, Oct 25, 2024 at 11:02 AM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Fri, 25 Oct 2024 at 19:36, Joanne Koong <joannelkoong@xxxxxxxxx> wrote: > > > That's a great point. It seems like we can just skip waiting on > > writeback to finish for fuse folios in sync(2) altogether then. I'll > > look into what's the best way to do this. > > I just tested this, and it turns out this doesn't quite work the way > I'd expected. I can trigger sync(2) being blocked by a suspended fuse > server: > > task:kworker/u16:3 state:D stack:0 pid:172 tgid:172 ppid:2 > flags:0x00004000 > Workqueue: writeback wb_workfn (flush-0:30) > Call Trace: > __schedule+0x40b/0xad0 > schedule+0x36/0x120 > inode_sleep_on_writeback+0x9d/0xb0 > wb_writeback+0x104/0x3d0 > wb_workfn+0x325/0x490 > process_one_work+0x1d8/0x520 > worker_thread+0x1af/0x390 > kthread+0xcc/0x100 > ret_from_fork+0x2d/0x50 > ret_from_fork_asm+0x1a/0x30 > > task:dd state:S stack:0 pid:1364 tgid:1364 > ppid:1336 flags:0x00000002 > Call Trace: > __schedule+0x40b/0xad0 > schedule+0x36/0x120 > request_wait_answer+0x16b/0x200 > __fuse_simple_request+0xd6/0x290 > fuse_flush_times+0x119/0x140 > fuse_write_inode+0x6d/0xc0 > __writeback_single_inode+0x36d/0x480 > writeback_single_inode+0xa8/0x170 > write_inode_now+0x75/0xa0 > fuse_flush+0x85/0x1c0 > filp_flush+0x2c/0x70 > __x64_sys_close+0x2e/0x80 > do_syscall_64+0x64/0x140 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > > task:sync state:D stack:0 pid:1365 tgid:1365 > ppid:1336 flags:0x00004002 > Call Trace: > __schedule+0x40b/0xad0 > schedule+0x36/0x120 > wb_wait_for_completion+0x56/0x80 > sync_inodes_sb+0xc5/0x450 > iterate_supers+0x69/0xd0 > ksys_sync+0x40/0xa0 > __do_sys_sync+0xa/0x20 > do_syscall_64+0x64/0x140 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > Thanks for the trace. If i'm understanding it correctly, this only blocks temporarily until the writeback wb_workfn is rescheduled? > Maybe I'm too paranoid about this, and in practice we can just let > sync(2) block in any case. But I want to understand this better in the case of a malicious fuse server, it could block sync(2) forever so I think this means we have to skip the wait for fuse folios altogether. Thanks, Joanne > first. > > Thanks, > Miklos
