fuse: add kernel-enforced request timeout option

There are situations where fuse servers can become unresponsive or
stuck, for example if the server is in a deadlock. Currently, there's
no good way to detect if a server is stuck and needs to be killed
manually.

This patchset adds a timeout option where if the server does not reply to a
request by the time the timeout elapses, the connection will be aborted.
This patchset also adds two dynamically configurable fuse sysctls
"default_request_timeout" and "max_request_timeout" for controlling/enforcing
timeout behavior system-wide.

Existing systems running fuse servers will not be affected unless they
explicitly opt into the timeout.

v8:
https://lore.kernel.org/linux-fsdevel/20241011191320.9159...
Changes from v8 -> v9:
* Fix comment for u16 fs_parse_result, ULONG_MAX instead of U32_MAX, fix
  spacing (Bernd)

v7:
https://lore.kernel.org/linux-fsdevel/20241007184258.2837...
Changes from v7 -> v8:
* Use existing lists for checking expirations (Miklos)

v6:
https://lore.kernel.org/linux-fsdevel/20240830162649.3849...
Changes from v6 -> v7:
- Make timer per-connection instead of per-request (Miklos)
- Make default granularity of time minutes instead of seconds
- Removed the reviewed-bys since the interface of this has changed (now
  minutes, instead of seconds)

v5:
https://lore.kernel.org/linux-fsdevel/20240826203234.4079...
Changes from v5 -> v6:
- Gate sysctl.o behind CONFIG_SYSCTL in makefile (kernel test robot)
- Reword/clarify last sentence in cover letter (Miklos)

v4:
https://lore.kernel.org/linux-fsdevel/20240813232241.2369...
Changes from v4 -> v5:
- Change timeout behavior from aborting request to aborting connection
  (Miklos)
- Clarify wording for sysctl documentation (Jingbo)

v3:
https://lore.kernel.org/linux-fsdevel/20240808190110.3188...
Changes from v3 -> v4:
- Fix wording on some comments to make it more clear
- Use simpler logic for timer (eg remove extra if checks, use mod timer API)
  (Josef)
- Sanity-check should be on FR_FINISHING not FR_FINISHED (Jingbo)
- Fix comment for "processing queue", add req->fpq = NULL safeguard  (Bernd)

v2:
https://lore.kernel.org/linux-fsdevel/20240730002348.3431...
Changes from v2 -> v3:
- Disarm / rearm timer in dev_do_read to handle race conditions (Bernrd)
- Disarm timer in error handling for fatal interrupt (Yafang)
- Clean up do_fuse_request_end (Jingbo)
- Add timer for notify retrieve requests 
- Fix kernel test robot errors for #define no-op functions

v1:
https://lore.kernel.org/linux-fsdevel/20240717213458.1613...
Changes from v1 -> v2:
- Add timeout for background requests
- Handle resend race condition
- Add sysctls

Joanne Koong (3):
  fs_parser: add fsparam_u16 helper
  fuse: add optional kernel-enforced timeout for requests
  fuse: add default_request_timeout and max_request_timeout sysctls

 Documentation/admin-guide/sysctl/fs.rst | 27 +++++++++
 fs/fs_parser.c                          | 14 +++++
 fs/fuse/dev.c                           | 80 +++++++++++++++++++++++++
 fs/fuse/fuse_i.h                        | 31 ++++++++++
 fs/fuse/inode.c                         | 33 ++++++++++
 fs/fuse/sysctl.c                        | 20 +++++++
 include/linux/fs_parser.h               |  9 ++-
 7 files changed, 211 insertions(+), 3 deletions(-)

-- 
2.43.5