Hello Milton, On Friday, November 14, 2008 you wrote:
> On Nov 13, 2008, at 10:32 PM, Yuri Tikhonov wrote: >> On Tuesday, November 11, 2008 Milton Miller wrote: >>>>>> #ifdef CONFIG_PTE_64BIT >>>>>> typedef unsigned long long pte_basic_t; >>>>>> +#ifdef CONFIG_PPC_256K_PAGES >>>>>> +#define PTE_SHIFT (PAGE_SHIFT - 7) >>>>> >>>>> This seems to be missing the comment on how many ptes are actually >>>>> in >>>>> the page that are in the other if and else cases. >>>> >>>> Ok. I'll fix this. Actually it's another hack: we don't use full page >>>> for PTE table because we need to reserve something for PGD >> >>> I don't understand "we need to reserve something for PGD". Do you >>> mean that you would not require a second page for the PGD because the >>> full pagetable could fit in one page? > ... >>> That does imply you want to allocate the pte page from a slab instead >>> of pgalloc. Is that covered? >> >> Well, in case of 256K PAGE_SIZE we do not need the PGD level indeed >> (18 bits are used for offset, and remaining 14 bits are for PTE index >> inside the PTE table). Even the full 256K PTE page isn't necessary to >> cover the full range: only half of it would be enough (with 14 bits we >> can address only 16K PTEs). >> >> But the head_44x.S code is essentially based on the assumption of >> 2-level page addressing. Also, I may guess that eliminating of the >> PGD level won't be as easy as just a re-implementation of the TLB-miss >> handlers in head_44x.S. So, the current approach for 256K-pages >> support was just a compromise between the required for the project >> functionality, and the effort necessary to achieve it. > So are you allocating the < PAGE_SIZE levels from slabs (either kmalloc > or dedicated) instead of allocating pages? Or are you wasting the > extra space? Wasting the extra space has a place here. > At a very minimum you need to comment this in the code. If I were > maintiner I would say not wasting large fractions of pages when the > page size is 256k would be my merge requirement. As I said, I'm fine > with keeping the page table two levels, but the tradeoff needs to be > documented. Agree, we'll document this fact, and re-submit the patch. Regards, Yuri -- Yuri Tikhonov, Senior Software Engineer Emcraft Systems, www.emcraft.com _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
