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

[Glauber Costa <glommer@parallels.com>]

On 09/04/2012 02:34 PM, Ezequiel Garcia wrote:
> 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.
> 

Ok. Slob also goes to the page allocator for higher order kmallocs,
so it should work.





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