[PATCH v2] xfs: enable WQ_MEM_RECLAIM on m_sync_workqueue

[Mike Snitzer <snitzer@kernel.org>]

The need for this fix was exposed while developing a new NFS feature
called "localio" which bypasses the network, if both the client and
server are on the same host, see:
https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/log/?h=nfs-localio-for-6.11

Because NFS's nfsiod_workqueue enables WQ_MEM_RECLAIM, writeback will
call into NFS and if localio is enabled the NFS client will call
directly into xfs_file_write_iter, this causes the following
backtrace when running xfstest generic/476 against NFS with localio:

  workqueue: WQ_MEM_RECLAIM writeback:wb_workfn is flushing !WQ_MEM_RECLAIM xfs-sync/vdc:xfs_flush_inodes_worker
  WARNING: CPU: 6 PID: 8525 at kernel/workqueue.c:3706 check_flush_dependency+0x2a4/0x328
  Modules linked in:
  CPU: 6 PID: 8525 Comm: kworker/u71:5 Not tainted 6.10.0-rc3-ktest-00032-g2b0a133403ab #18502
  Hardware name: linux,dummy-virt (DT)
  Workqueue: writeback wb_workfn (flush-0:33)
  pstate: 400010c5 (nZcv daIF -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
  pc : check_flush_dependency+0x2a4/0x328
  lr : check_flush_dependency+0x2a4/0x328
  sp : ffff0000c5f06bb0
  x29: ffff0000c5f06bb0 x28: ffff0000c998a908 x27: 1fffe00019331521
  x26: ffff0000d0620900 x25: ffff0000c5f06ca0 x24: ffff8000828848c0
  x23: 1fffe00018be0d8e x22: ffff0000c1210000 x21: ffff0000c75fde00
  x20: ffff800080bfd258 x19: ffff0000cad63400 x18: ffff0000cd3a4810
  x17: 0000000000000000 x16: 0000000000000000 x15: ffff800080508d98
  x14: 0000000000000000 x13: 204d49414c434552 x12: 1fffe0001b6eeab2
  x11: ffff60001b6eeab2 x10: dfff800000000000 x9 : ffff60001b6eeab3
  x8 : 0000000000000001 x7 : 00009fffe491154e x6 : ffff0000db775593
  x5 : ffff0000db775590 x4 : ffff0000db775590 x3 : 0000000000000000
  x2 : 0000000000000027 x1 : ffff600018be0d62 x0 : dfff800000000000
  Call trace:
   check_flush_dependency+0x2a4/0x328
   __flush_work+0x184/0x5c8
   flush_work+0x18/0x28
   xfs_flush_inodes+0x68/0x88
   xfs_file_buffered_write+0x128/0x6f0
   xfs_file_write_iter+0x358/0x448
   nfs_local_doio+0x854/0x1568
   nfs_initiate_pgio+0x214/0x418
   nfs_generic_pg_pgios+0x304/0x480
   nfs_pageio_doio+0xe8/0x240
   nfs_pageio_complete+0x160/0x480
   nfs_writepages+0x300/0x4f0
   do_writepages+0x12c/0x4a0
   __writeback_single_inode+0xd4/0xa68
   writeback_sb_inodes+0x470/0xcb0
   __writeback_inodes_wb+0xb0/0x1d0
   wb_writeback+0x594/0x808
   wb_workfn+0x5e8/0x9e0
   process_scheduled_works+0x53c/0xd90
   worker_thread+0x370/0x8c8
   kthread+0x258/0x2e8
   ret_from_fork+0x10/0x20

Fix this by enabling WQ_MEM_RECLAIM on XFS's m_sync_workqueue.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/xfs/xfs_super.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

[v2: dropped RFC, this fixes xfstests generic/476, resubmitting with more feeling]

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 27e9f749c4c7..dbe6af00708b 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -574,7 +574,8 @@ xfs_init_mount_workqueues(
 		goto out_destroy_blockgc;
 
 	mp->m_sync_workqueue = alloc_workqueue("xfs-sync/%s",
-			XFS_WQFLAGS(WQ_FREEZABLE), 0, mp->m_super->s_id);
+			XFS_WQFLAGS(WQ_FREEZABLE | WQ_MEM_RECLAIM),
+			0, mp->m_super->s_id);
 	if (!mp->m_sync_workqueue)
 		goto out_destroy_inodegc;
 
-- 
2.44.0