Re: [PATCH v2] mm, slob: Drop usage of page->private for storing page-sized allocations

[Ezequiel Garcia <elezegarcia@gmail.com>]

Hi Glauber,

On Tue, Sep 4, 2012 at 7:15 AM, Glauber Costa <glommer@parallels.com> wrote:
> On 09/04/2012 02:13 PM, Ezequiel Garcia wrote:
>> This field was being used to store size allocation so it could be
>> retrieved by ksize(). However, it is a bad practice to not mark a page
>> as a slab page and then use fields for special purposes.
>> There is no need to store the allocated size and
>> ksize() can simply return PAGE_SIZE << compound_order(page).
>
> What happens for allocations smaller than a page?
> It seems you are breaking ksize for those.
>

Allocations smaller than a page save its own size on a header
located at each returned pointer. This is documented at the beginning of slob:

"Above this is an implementation of kmalloc/kfree. Blocks returned
from kmalloc are prepended with a 4-byte header with the kmalloc size."

For this objects (smaller than a page) ksize works fine:

size_t ksize(const void *block) {
[...]
  unsigned int *m = (unsigned int *)(block - align);
  return SLOB_UNITS(*m) * SLOB_UNIT;

(see how ksize substract 'align' from block to find the header?)

Of course, it's possible I've overlooked something, but I think this
should work.

Thanks!
Ezequiel.

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