Linux: Never sleep in kmem_cache_alloc(..., KM_NOSLEEP) (openzfs#14926)

When a kmem cache is exhausted and needs to be expanded a new
slab is allocated.  KM_SLEEP callers can block and wait for the
allocation, but KM_NOSLEEP callers were incorrectly allowed to
block as well.

Resolve this by attempting an emergency allocation as a best
effort.  This may fail but that's fine since any KM_NOSLEEP
consumer is required to handle an allocation failure.

Signed-off-by: Brian Behlendorf <[email protected]>
Reviewed-by: Adam Moss <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Richard Yao <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>

module/os/linux/spl/spl-kmem-cache.c

@@ -1015,9 +1015,19 @@ spl_cache_grow(spl_kmem_cache_t *skc, int flags, void **obj)
 	ASSERT0(flags & ~KM_PUBLIC_MASK);
 	ASSERT(skc->skc_magic == SKC_MAGIC);
 	ASSERT((skc->skc_flags & KMC_SLAB) == 0);
-	might_sleep();
+
 	*obj = NULL;
 
+	/*
+	 * Since we can't sleep attempt an emergency allocation to satisfy
+	 * the request.  The only alterative is to fail the allocation but
+	 * it's preferable try.  The use of KM_NOSLEEP is expected to be rare.
+	 */
+	if (flags & KM_NOSLEEP)
+		return (spl_emergency_alloc(skc, flags, obj));
+
+	might_sleep();
+
 	/*
 	 * Before allocating a new slab wait for any reaping to complete and
 	 * then return so the local magazine can be rechecked for new objects.