Re: [PATCH v2 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, 25 Oct 2024 at 19:36, Joanne Koong <joannelkoong@gmail.com> 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

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
first.

Thanks,
Miklos