NFS/localio: issue IO inline when not in a memory-reclaim context

Every LOCALIO read and write is currently bounced through the dedicated
!WQ_MEM_RECLAIM nfslocaliod_workqueue.  That bounce is only actually
required when the submitting context is a memory-reclaim context: LOCALIO
issues IO directly into a stacked local filesystem (e.g. XFS) which may in
turn flush its own !WQ_MEM_RECLAIM workqueue.  Doing that from a
WQ_MEM_RECLAIM worker (most importantly writeback's wb_workfn on bdi_wq) or
an explicit PF_MEMALLOC reclaim task trips check_flush_dependency() and
risks a forward-progress deadlock, which is why commit b9f5dd57f4a5
("nfs/localio: use dedicated workqueues for filesystem read and write")
introduced the intermediate workqueue.

Outside of reclaim context -- ordinary application/task submission such as
O_DIRECT or fsync-driven writeback -- the workqueue hop buys nothing and
merely adds a context switch and scheduling latency per IO while discarding
the NFS client's inherent application-context parallelism.

Add current_is_workqueue_mem_reclaim(), which reports whether %current is a
WQ_MEM_RECLAIM worker using the same predicate check_flush_dependency()
warns on.  Use it, together with the PF_MEMALLOC check, in the new
nfs_local_defer_io() helper to decide per-IO whether nfs_local_do_read()
and nfs_local_do_write() must defer to nfslocaliod_workqueue or may issue
the IO inline.  Buffered writeback continues to bounce (wb_workfn is a
WQ_MEM_RECLAIM worker); O_DIRECT and app-context submission now run inline.

Running nfs_local_call_write() inline is safe: it already saves and
restores current->flags around the PF_LOCAL_THROTTLE|PF_MEMALLOC_NOIO it
sets and scopes the file opener's creds.  The async O_DIRECT completion
path is likewise unaffected: when the underlying filesystem returns
-EIOCBQUEUED, the kiocb ki_complete callback (nfs_local_read_aio_complete /
nfs_local_write_aio_complete) can run in bottom-half context and so must
still defer the pgio completion (nfs_local_pgio_release -> rpc_call_done) to
nfsiod_workqueue via nfs_local_pgio_aio_complete().  That completion hop is
independent of how the IO was submitted, and this change leaves it as-is;
only the submission side stops unconditionally hopping through
nfslocaliod_workqueue.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a283766a192a..c8a36423cb34 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -661,6 +661,7 @@ extern void workqueue_set_min_active(struct workqueue_struct *wq,
 				     int min_active);
 extern struct work_struct *current_work(void);
 extern bool current_is_workqueue_rescuer(void);
+extern bool current_is_workqueue_mem_reclaim(void);
 extern bool workqueue_congested(int cpu, struct workqueue_struct *wq);
 extern unsigned int work_busy(struct work_struct *work);
 extern __printf(1, 2) void set_worker_desc(const char *fmt, ...);