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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 29 Sep 2015 09:12:19 +0200

On Mon, 28 Sep 2015 11:30:00 -0500 (CDT) Christoph Lameter <[email-masked]> wrote:

> 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.

Okay, I will remove the comment.

The best doc on SLUB and SLAB layout comes from your slides titled:
 "Slab allocators in the Linux Kernel: SLAB, SLOB, SLUB"

Lets gracefully add a link to the slides here:
 http://events.linuxfoundation.org/sites/events/files/slides/slaballocators.pdf

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer