[PATCH -V1 06/24] powerpc: Reduce PTE table memory wastage

Aneesh Kumar K.V aneesh.kumar at linux.vnet.ibm.com
Wed Mar 6 15:01:58 EST 2013

Benjamin Herrenschmidt <benh at kernel.crashing.org> writes:

> On Mon, 2013-03-04 at 16:28 +0530, Aneesh Kumar K.V wrote:
>> I added the below comment when initializing the list.
>> +#ifdef CONFIG_PPC_64K_PAGES
>> +       /*
>> +        * Used to support 4K PTE fragment. The pages are added to list,
>> +        * when we have free framents in the page. We track the whether
>> +        * a page frament is available using page._mapcount. A value of
>> +        * zero indicate none of the fragments are used and page can be
>> +        * freed. A value of FRAG_MASK indicate all the fragments are used
>> +        * and hence the page will be removed from the below list.
>> +        */
>> +       INIT_LIST_HEAD(&init_mm.context.pgtable_list);
>> +#endif
>> I am not sure about why you say there is no consistent rule. Can you
>> elaborate on that ?
> Do you really need that list ? I assume it's meant to allow you to find
> free frags when allocating but my worry is that you'll end up losing
> quite a bit of node locality of PTE pages....
> It may or may not work but can you investigate doing things differently
> here ? The idea I want you to consider is to always allocate a full
> page, but make the relationship of the fragments to PTE pages fixed. IE.
> the fragment in the page is a function of the VA.
> Basically, the algorithm for allocation is roughly:
>  - Walk the tree down to the PMD ptr (* that can be improved with a
> generic change, see below)
>  - Check if any of the neighbouring PMDs is populated. If yes, you have
> your page and pick the appropriate fragment based on the VA
>  - If not, allocate and populate
> On free, similarly, you checked if all neighbouring PMDs have been
> cleared, in which case you can fire off the page for RCU freeing.
> (*) By changing pte_alloc_one to take the PMD ptr (which the call side
> has right at hand) you can avoid the tree lookup.

Will try this.


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