[PATCH 4/7] Store huge page linux pte in mm_struct

[carson bill]

2012/2/6, Catalin Marinas <catalin.marinas at arm.com>:
> bill4carson wrote:
>> Normal page based pte has hardware version and linux version, these
>> two kinds of ptes occupy half of 4K page, and each of two pmd level
>> entry pointer to this half 4K page.
>>
>> For huge page, mappings only exist in pmd level from hardware
>> perspective, however mm subsystem also needs to know the linux version
>> *pte* of this huge page based mapping, problems is where to store
>> these huge page linux pte?
>>
>> A:
>> Store huge page linux pte in mm_struct
>>
>> B:
>> Store huge page linux pte in mm_context_t suggested by Catalin.
>> This is almost like option A, while it's nice to modify arm code only
>> and take small effort to make it happen.
>>
>> C:
>> Modify pgd_alloc to allocate 2048 pgd_t entries plus two void pointers
>> Use these two additional pointers to store huge page linux pte.
>> It's feasible, but I don't know whether this is a good idea.
>
> I wouldn't add this to pgd_alloc since not all tasks need huge pages.
> First option is also not feasible as we shouldn't touch generic code.
> This leaves us with B, unless better options are suggested.
>

So far B sounds better, I will make that happens in V2.
I don't know whether other guru have any opinion on this patch,
Advice/suggestion/criticism are all truly welcome!


> I had a patch couple of years ago
> (http://article.gmane.org/gmane.linux.ports.arm.kernel/55045) to use the
> AF (Access Flag) bit and free up some bits in the page table, allowing
> us to drop the Linux PTE. But we end up in a bigger configuration mess
> as we already have to choose between classic MMU and LPAE. The
> performance advantage wasn't that big either.
> --
> Catalin
>