mempool: add error injection support

Add a call to should_fail_ex that forces mempool to actually allocate
from the pool to stress the mempool implementation when enabled through
debugfs.  By default should_fail{,_ex} prints a very verbose stack trace
that clutters the kernel log, slows down execution and triggers the
kernel bug detection in xfstests.  Pass FAULT_NOWARN and print a
single-line message notating the caller instead so that full tests
can be run with fault injection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://patch.msgid.link/20251113084022.1255121-5-hch@lst.de
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Christoph Hellwig

mm/mempool.c

@@ -9,7 +9,7 @@
  *  started by Ingo Molnar, Copyright (C) 2001
  *  debugging by David Rientjes, Copyright (C) 2015
  */
-
+#include <linux/fault-inject.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
 #include <linux/highmem.h>
@@ -20,6 +20,15 @@
 #include <linux/writeback.h>
 #include "slab.h"
 
+static DECLARE_FAULT_ATTR(fail_mempool_alloc);
+
+static int __init mempool_faul_inject_init(void)
+{
+	return PTR_ERR_OR_ZERO(fault_create_debugfs_attr("fail_mempool_alloc",
+			NULL, &fail_mempool_alloc));
+}
+late_initcall(mempool_faul_inject_init);
+
 #ifdef CONFIG_SLUB_DEBUG_ON
 static void poison_error(mempool_t *pool, void *element, size_t size,
 			 size_t byte)
@@ -404,9 +413,15 @@ void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
 	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
 
 repeat_alloc:
+	if (should_fail_ex(&fail_mempool_alloc, 1, FAULT_NOWARN)) {
+		pr_info("forcing mempool usage for %pS\n",
+				(void *)_RET_IP_);
+		element = NULL;
+	} else {
+		element = pool->alloc(gfp_temp, pool->pool_data);
+	}
 
-	element = pool->alloc(gfp_temp, pool->pool_data);
-	if (likely(element != NULL))
+	if (likely(element))
 		return element;
 
 	spin_lock_irqsave(&pool->lock, flags);