[RFC PATCH] xfs: enable WQ_MEM_RECLAIM on m_sync_workqueue

[Mike Snitzer <snitzer@kernel.org>]

I'm developing an NFS feature called "localio" that 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:

00573 workqueue: WQ_MEM_RECLAIM writeback:wb_workfn is flushing !WQ_MEM_RECLAIM xfs-sync/vdc:xfs_flush_inodes_worker
00573 WARNING: CPU: 6 PID: 8525 at kernel/workqueue.c:3706 check_flush_dependency+0x2a4/0x328
00573 Modules linked in:
00573 CPU: 6 PID: 8525 Comm: kworker/u71:5 Not tainted 6.10.0-rc3-ktest-00032-g2b0a133403ab #18502
00573 Hardware name: linux,dummy-virt (DT)
00573 Workqueue: writeback wb_workfn (flush-0:33)
00573 pstate: 400010c5 (nZcv daIF -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
0573 pc : check_flush_dependency+0x2a4/0x328
00573 lr : check_flush_dependency+0x2a4/0x328
00573 sp : ffff0000c5f06bb0
00573 x29: ffff0000c5f06bb0 x28: ffff0000c998a908 x27: 1fffe00019331521
00573 x26: ffff0000d0620900 x25: ffff0000c5f06ca0 x24: ffff8000828848c0
00573 x23: 1fffe00018be0d8e x22: ffff0000c1210000 x21: ffff0000c75fde00
00573 x20: ffff800080bfd258 x19: ffff0000cad63400 x18: ffff0000cd3a4810
00573 x17: 0000000000000000 x16: 0000000000000000 x15: ffff800080508d98
00573 x14: 0000000000000000 x13: 204d49414c434552 x12: 1fffe0001b6eeab2
00573 x11: ffff60001b6eeab2 x10: dfff800000000000 x9 : ffff60001b6eeab3
00573 x8 : 0000000000000001 x7 : 00009fffe491154e x6 : ffff0000db775593
00573 x5 : ffff0000db775590 x4 : ffff0000db775590 x3 : 0000000000000000
00573 x2 : 0000000000000027 x1 : ffff600018be0d62 x0 : dfff800000000000
00573 Call trace:
00573  check_flush_dependency+0x2a4/0x328
00573  __flush_work+0x184/0x5c8
00573  flush_work+0x18/0x28
00573  xfs_flush_inodes+0x68/0x88
00573  xfs_file_buffered_write+0x128/0x6f0
00573  xfs_file_write_iter+0x358/0x448
00573  nfs_local_doio+0x854/0x1568
00573  nfs_initiate_pgio+0x214/0x418
00573  nfs_generic_pg_pgios+0x304/0x480
00573  nfs_pageio_doio+0xe8/0x240
00573  nfs_pageio_complete+0x160/0x480
00573  nfs_writepages+0x300/0x4f0
00573  do_writepages+0x12c/0x4a0
00573  __writeback_single_inode+0xd4/0xa68
00573  writeback_sb_inodes+0x470/0xcb0
00573  __writeback_inodes_wb+0xb0/0x1d0
00573  wb_writeback+0x594/0x808
00573  wb_workfn+0x5e8/0x9e0
00573  process_scheduled_works+0x53c/0xd90
00573  worker_thread+0x370/0x8c8
00573  kthread+0x258/0x2e8
00573  ret_from_fork+0x10/0x20

Fix this by enabling WQ_MEM_RECLAIM on XFS's m_sync_workqueue.

It could be this change is a non-starter due to WQ_MEM_RECLAIM no
longer allowing m_sync_workqueue to be "sync"?  All comments and
suggestions welcome!

Signed-off-by: Mike Snitzer <snitzer@kernel.org>

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;