Re: [PATCH 5/7] slub: support for bulk free with SLUB freelists

[Christoph Lameter <cl@linux.com>]

On Mon, 28 Sep 2015, Jesper Dangaard Brouer wrote:

> Not knowing SLUB as well as you, it took me several hours to realize
> init_object() didn't overwrite the freepointer in the object.  Thus, I
> think these comments make the reader aware of not-so-obvious
> side-effects of SLAB_POISON and SLAB_RED_ZONE.

>From the source:

/*
 * Object layout:
 *
 * object address
 *      Bytes of the object to be managed.
 *      If the freepointer may overlay the object then the free
 *      pointer is the first word of the object.
 *
 *      Poisoning uses 0x6b (POISON_FREE) and the last byte is
 *      0xa5 (POISON_END)
 *
 * object + s->object_size
 *      Padding to reach word boundary. This is also used for Redzoning.
 *      Padding is extended by another word if Redzoning is enabled and
 *      object_size == inuse.
 *
 *      We fill with 0xbb (RED_INACTIVE) for inactive objects and with
 *      0xcc (RED_ACTIVE) for objects in use.
 *
 * object + s->inuse
 *      Meta data starts here.
 *
 *      A. Free pointer (if we cannot overwrite object on free)
 *      B. Tracking data for SLAB_STORE_USER
 *      C. Padding to reach required alignment boundary or at mininum
 *              one word if debugging is on to be able to detect writes
 *              before the word boundary.
 *
 *      Padding is done using 0x5a (POISON_INUSE)
 *
 * object + s->size
 *      Nothing is used beyond s->size.
 *
 * If slabcaches are merged then the object_size and inuse boundaries are
mostly
 * ignored. And therefore no slab options that rely on these boundaries
 * may be used with merged slabcaches.
 */

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