flex_array: poison free elements

Newly initialized flex_array's and/or flex_array_part's are now poisoned
with a new poison value, FLEX_ARRAY_FREE.  It's value is similar to
POISON_FREE used in the various slab allocators, but is different to
distinguish between flex array's poisoned kmem and slab allocator poisoned
kmem.

This will allow us to identify flex_array_part's that only contain free
elements (and free them with an addition to the flex_array API).  This
could also be extended in the future to identify `get' uses on elements
that have not been `put'.

If __GFP_ZERO is passed for a part's gfp mask, the poisoning is avoided.
These elements are considered to be in-use since they have been
initialized.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 8 deletions

diff --git a/lib/flex_array.c b/lib/flex_array.c
index b68f99be408..e22d0e9776a 100644
--- a/lib/flex_array.c
+++ b/lib/flex_array.c
@@ -113,6 +113,8 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
                 return NULL;
         ret->element_size = element_size;
         ret->total_nr_elements = total;
+        if (elements_fit_in_base(ret) && !(flags & __GFP_ZERO))
+                memset(ret->parts[0], FLEX_ARRAY_FREE, bytes_left_in_base());
         return ret;
 }
 
@@ -159,15 +161,12 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
 {
         struct flex_array_part *part = fa->parts[part_nr];
         if (!part) {
-                /*
-                 * This leaves the part pages uninitialized
-                 * and with potentially random data, just
-                 * as if the user had kmalloc()'d the whole.
-                 * __GFP_ZERO can be used to zero it.
-                 */
-                part = kmalloc(FLEX_ARRAY_PART_SIZE, flags);
+                part = kmalloc(sizeof(struct flex_array_part), flags);
                 if (!part)
                         return NULL;
+                if (!(flags & __GFP_ZERO))
+                        memset(part, FLEX_ARRAY_FREE,
+                                sizeof(struct flex_array_part));
                 fa->parts[part_nr] = part;
         }
         return part;
@@ -228,7 +227,7 @@ int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
                         return -EINVAL;
         }
         dst = &part->elements[index_inside_part(fa, element_nr)];
-        memset(dst, 0, fa->element_size);
+        memset(dst, FLEX_ARRAY_FREE, fa->element_size);
         return 0;
 }