[PATCH] mm: make should_failslab always available for fault injection

[Howard McLauchlan <hmclauchlan@fb.com>]

should_failslab() is a convenient function to hook into for directed
error injection into kmalloc(). However, it is only available if a
config flag is set.

The following BCC script, for example, fails kmalloc() calls after a
btrfs umount:

from bcc import BPF

prog = r"""
BPF_HASH(flag);

int kprobe__btrfs_close_devices(void *ctx) {
        u64 key = 1;
        flag.update(&key, &key);
        return 0;
}

int kprobe__should_failslab(struct pt_regs *ctx) {
        u64 key = 1;
        u64 *res;
        res = flag.lookup(&key);
        if (res != 0) {
            bpf_override_return(ctx, -ENOMEM);
        }
        return 0;
}
"""
b = BPF(text=prog)

while 1:
    b.kprobe_poll()

This patch refactors the should_failslab implementation so that the
function is always available for error injection, independent of flags.

This change would be similar in nature to commit f5490d3ec921 ("block:
Add should_fail_bio() for bpf error injection").

Signed-off-by: Howard McLauchlan <hmclauchlan@fb.com>
---
 include/linux/fault-inject.h | 5 +++--
 mm/failslab.c                | 2 +-
 mm/slab_common.c             | 8 ++++++++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/fault-inject.h b/include/linux/fault-inject.h
index c3c95d18bf43..7e6c77740413 100644
--- a/include/linux/fault-inject.h
+++ b/include/linux/fault-inject.h
@@ -64,10 +64,11 @@ static inline struct dentry *fault_create_debugfs_attr(const char *name,
 
 struct kmem_cache;
 
+int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
 #ifdef CONFIG_FAILSLAB
-extern bool should_failslab(struct kmem_cache *s, gfp_t gfpflags);
+extern bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags);
 #else
-static inline bool should_failslab(struct kmem_cache *s, gfp_t gfpflags)
+static inline bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
 {
 	return false;
 }
diff --git a/mm/failslab.c b/mm/failslab.c
index 8087d976a809..1f2f248e3601 100644
--- a/mm/failslab.c
+++ b/mm/failslab.c
@@ -14,7 +14,7 @@ static struct {
 	.cache_filter = false,
 };
 
-bool should_failslab(struct kmem_cache *s, gfp_t gfpflags)
+bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
 {
 	/* No fault-injection for bootstrap cache */
 	if (unlikely(s == kmem_cache))
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 10f127b2de7c..e99884490f14 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1532,3 +1532,11 @@ EXPORT_TRACEPOINT_SYMBOL(kmalloc_node);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc_node);
 EXPORT_TRACEPOINT_SYMBOL(kfree);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
+
+int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
+{
+	if (__should_failslab(s, gfpflags))
+		return -ENOMEM;
+	return 0;
+}
+ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
-- 
2.14.1

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