[PATCH 2/7] Add various hugetlb page table fix

[carson bill]

2012/2/7, Catalin Marinas <catalin.marinas at arm.com>:
> On Tue, Feb 07, 2012 at 01:42:01AM +0000, bill4carson wrote:
>> On 2012年02月07日 00:26, Catalin Marinas wrote:
>> > On Wed, Feb 01, 2012 at 03:10:21AM +0000, bill4carson wrote:
>> >> Why L_PTE_HUGEPAGE is needed?
>> >>
>> >> hugetlb subsystem will call pte_page to derive the corresponding page
>> >> struct from a given pte, and pte_pfn is used first to convert pte into
>> >> a page frame number.
>> >
>> > Are you sure the pte_pfn() conversion is right? Does it need to be
>> > different from the 4K pfn?
> ...
>> pte_page is defined as following to derive page struct from a given pte.
>> This macro is used both in generic mm as well as hugetlb sub-system, so
>> we need do the switch in pte_pfn to mark huge page based linux pte out
>> of normal page based linux pte, that's what L_PTE_HUGEPAGE for.
>>
>> #define pte_page(pte)		pfn_to_page(pte_pfn(pte))
>>
>> So L_PTE_HUGEPAGE is *NOT* set in normal page based linux pte,
>> linux pte bits[31:12] is the page frame number;
>
> I agree.
>
>> otherwise, we got a huge page based linux pte, and linux pte
>> bits[31:20] is page frame number for SECTION mapping, and bits[31:24]
>> is page frame number for SUPER-SECTION mapping.
>
> Actually it is still 31:12 but with bits 19:12 or 23:12 masked out. So
> you do the correct shift by PAGE_SHIFT with the additional masking for
> huge pages (harmless).
>
> But do we actually need this masking? Do the huge_pte_offset() or
> huge_pte_alloc() functions return the Linux pte (pmd) for the huge page?
> If yes, can we not ensure that bits 19:12 are already zero? This
> shouldn't be any different from the 4K Linux pte but with an address
> aligned to 1MB.
>

I'm afraid there is some misunderstanding.
huge_pte_offset() returns the huge linux pte address if they exist;
huge_pte_alloc()  allocates a location to store huge linux pte, and
return this address;
non of above functions return huge linux pte *value*.

make_huge_pte() will return huge linux pte for a given page and vma
protection bits,
please notice pte_mkhuge is used to mark this pte as huge linux pte by setting
L_PTE_HUGEPAGE, then set_huge_pte_at() is used to set huge linux pte as well
huge hardware pte.


2113static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page,
2114                                int writable)
2115{
2116        pte_t entry;
2117
2118        if (writable) {
2119                entry =
2120                    pte_mkwrite(pte_mkdirty(mk_pte(page,
vma->vm_page_prot)));
2121        } else {
2122                entry = huge_pte_wrprotect(mk_pte(page, vma->vm_page_prot));
2123        }
2124        entry = pte_mkyoung(entry);
2125        entry = pte_mkhuge(entry);
2126
2127        return entry;
2128}

Hence, normal linux pte must has L_PTE_HUGEPAE cleared;
A huge linux pte must has L_PTE_HUGEPAGE(BIT11) set
This could lead to L_PTE_HPAGE_2M(BIT12) or L_PTE_HPAGE_16M(BIT13) set
respectively, that's why the masking is needed for pte_pfn.


+#ifdef CONFIG_ARM_HUGETLB_SUPPORT
+#define L_PTE_HUGEPAGE	(_AT(pteval_t, 1) << 11) /* mark hugepage */
+#define L_PTE_HPAGE_2M  (_AT(pteval_t, 1) << 12) /* only when HUGEPAGE set */
+#define L_PTE_HPAGE_16M (_AT(pteval_t, 1) << 13) /* only when HUGEPAGE set */
+#endif


> --
> Catalin
>