percpu: make allocation failures more verbose

Warn and dump stack when percpu allocation fails.  percpu allocator is
still young and unchecked NULL percpu pointer usage can result in
random memory corruption when combined with the pointer shifting in
access macros.  Allocation failures should be rare and the warning
message will be disabled after certain times.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/mm/percpu.c b/mm/percpu.c
index 83617ca3ba527190c17d23b846245ad79c78ccb7..4a048abad0436d2bd883fd25395b1e44c0b6b99f 100644 (file)
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1043,7 +1043,9 @@ static struct pcpu_chunk *alloc_pcpu_chunk(void)
  */
 static void *pcpu_alloc(size_t size, size_t align, bool reserved)
 {
+       static int warn_limit = 10;
        struct pcpu_chunk *chunk;
+       const char *err;
        int slot, off;
 
        if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE)) {
@@ -1059,11 +1061,14 @@ static void *pcpu_alloc(size_t size, size_t align, bool reserved)
        if (reserved && pcpu_reserved_chunk) {
                chunk = pcpu_reserved_chunk;
                if (size > chunk->contig_hint ||
-                   pcpu_extend_area_map(chunk) < 0)
+                   pcpu_extend_area_map(chunk) < 0) {
+                       err = "failed to extend area map of reserved chunk";
                        goto fail_unlock;
+               }
                off = pcpu_alloc_area(chunk, size, align);
                if (off >= 0)
                        goto area_found;
+               err = "alloc from reserved chunk failed";
                goto fail_unlock;
        }
 
@@ -1080,6 +1085,7 @@ restart:
                        case 1:
                                goto restart;   /* pcpu_lock dropped, restart */
                        default:
+                               err = "failed to extend area map";
                                goto fail_unlock;
                        }
 
@@ -1093,8 +1099,10 @@ restart:
        spin_unlock_irq(&pcpu_lock);
 
        chunk = alloc_pcpu_chunk();
-       if (!chunk)
+       if (!chunk) {
+               err = "failed to allocate new chunk";
                goto fail_unlock_mutex;
+       }
 
        spin_lock_irq(&pcpu_lock);
        pcpu_chunk_relocate(chunk, -1);
@@ -1107,6 +1115,7 @@ area_found:
        if (pcpu_populate_chunk(chunk, off, size)) {
                spin_lock_irq(&pcpu_lock);
                pcpu_free_area(chunk, off);
+               err = "failed to populate";
                goto fail_unlock;
        }
 
@@ -1119,6 +1128,13 @@ fail_unlock:
        spin_unlock_irq(&pcpu_lock);
 fail_unlock_mutex:
        mutex_unlock(&pcpu_alloc_mutex);
+       if (warn_limit) {
+               pr_warning("PERCPU: allocation failed, size=%zu align=%zu, "
+                          "%s\n", size, align, err);
+               dump_stack();
+               if (!--warn_limit)
+                       pr_info("PERCPU: limit reached, disable warning\n");
+       }
        return NULL;
 }