[RFC PATCH 0/1] mm/filemap: make writeback wait killable in __filemap_fdatawait_range()

[Rushil Patel <rushil.patel@gsacapital.com>]

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We run Slurm on compute nodes with NFS mounts (NFSv4.1, NetApp).
When a job is cancelled, processes with dirty NFS pages get stuck
in D-state inside folio_wait_bit_common() because
__filemap_fdatawait_range() uses folio_wait_writeback(), which is
TASK_UNINTERRUPTIBLE. If the filer is slow to respond these processes are
unkillable - we've found the only recovery in practice is rebooting
the node.

The patch switches to folio_wait_writeback_killable() so SIGKILL can
interrupt the wait. Writeback itself continues on the server, we just stop
waiting for the ack. All 6 callers of __filemap_fdatawait_range() detect
errors independently via errseq_t / filemap_check_errors(), so the early
return doesn't suppress error reporting.

The tricky part is a re-entry through do_exit(). Making the wait killable
alone isn't enough - we hit this in testing:

  1. SIGKILL wakes the killable wait, signal is consumed by get_signal()
  2. do_exit() -> exit_signals() sets PF_EXITING
  3. do_exit() -> exit_files() -> nfs4_file_flush() -> nfs_wb_all()
     re-enters __filemap_fdatawait_range()
  4. wants_signal() checks PF_EXITING *before* the SIGKILL special case
     (kernel/signal.c:951 vs 954), so it returns false
  5. No signal can wake the second wait -> stuck in D-state again

The PF_EXITING check at the top of the function avoids re-entering the
wait entirely. This is the same pattern used in mm/oom_kill.c,
mm/memcontrol.c, block/blk-ioc.c, and io_uring/.

Reproduced with iptables DROP on port 2049, confirmed the killable-only
revision gets stuck on re-entry, and the PF_EXITING + killable revision
kills cleanly.

Sending as RFC because this touches the generic writeback sync path in
mm/filemap.c rather than being NFS-specific. NFS can't really fix this on
its own - it reaches __filemap_fdatawait_range() through
filemap_write_and_wait() and doesn't own the wait. But I wanted to get
guidance on whether this is the right place for the fix, or if you'd prefer
a different approach.

Best regards,

Rushil

Rushil Patel (1):
  mm/filemap: make writeback wait killable in
    __filemap_fdatawait_range()

 mm/filemap.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

-- 
2.47.3

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