Re: [Libhugetlbfs-devel] Buglet in 16G page handling

David Gibson
2008-09-03T00:20:33Z

On Tue, Sep 02, 2008 at 12:12:27PM -0500, Jon Tollefson wrote:
> David Gibson wrote:
> > When BenH and I were looking at the new code for handling 16G pages,
> > we noticed a small bug. It doesn't actually break anything user
> > visible, but it's certainly not the way things are supposed to be.
> > The 16G patches didn't update the huge_pte_offset() and
> > huge_pte_alloc() functions, which means that the hugepte tables for
> > 16G pages will be allocated much further down the page table tree than
> > they should be - allocating several levels of page table with a single
> > entry in them along the way.
> >
> > The patch below is supposed to fix this, cleaning up the existing
> > handling of 64k vs 16M pages while its at it. However, it needs some
> > testing.
> >
> > I've checked that it doesn't break existing 16M support, either with
> > 4k or 64k base pages. I haven't figured out how to test with 64k
> > pages yet, at least until the multisize support goes into
> > libhugetlbfs. For 16G pages, I just don't have access to a machine
> > with enough memory to test. Jon, presumably you must have found such
> > a machine when you did the 16G page support in the first place. Do
> > you still have access, and can you test this patch?
> >
> I do have access to a machine to test it. I applied the patch to -rc4
> and used a pseries_defconfig. I boot with
> default_hugepagesz=16G... in order to test huge page sizes other then
> 16M at this point.
>
> Running the libhugetlbfs test suite it gets as far as Readback (64):
> PASS
> before it hits the following program check.

Ah, yes, oops, forgot to fix up the pagetable freeing path in line
with the other changes. Try the revised version below.

Index: working-2.6/arch/powerpc/mm/hugetlbpage.c
===================================================================
--- working-2.6.orig/arch/powerpc/mm/hugetlbpage.c 2008-09-02 11:50:12.000000000 +1000
+++ working-2.6/arch/powerpc/mm/hugetlbpage.c 2008-09-03 10:10:54.000000000 +1000
@@ -128,29 +128,37 @@ static int __hugepte_alloc(struct mm_str
return 0;
}

-/* Base page size affects how we walk hugetlb page tables */
-#ifdef CONFIG_PPC_64K_PAGES
-#define hpmd_offset(pud, addr, h) pmd_offset(pud, addr)
-#define hpmd_alloc(mm, pud, addr, h) pmd_alloc(mm, pud, addr)
-#else
-static inline
-pmd_t *hpmd_offset(pud_t *pud, unsigned long addr, struct hstate *hstate)
+
+static pud_t *hpud_offset(pgd_t *pgd, unsigned long addr, struct hstate *hstate)
+{
+ if (huge_page_shift(hstate) < PUD_SHIFT)
+ return pud_offset(pgd, addr);
+ else
+ return (pud_t *) pgd;
+}
+static pud_t *hpud_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long addr,
+ struct hstate *hstate)
{
- if (huge_page_shift(hstate) == PAGE_SHIFT_64K)
+ if (huge_page_shift(hstate) < PUD_SHIFT)
+ return pud_alloc(mm, pgd, addr);
+ else
+ return (pud_t *) pgd;
+}
+static pmd_t *hpmd_offset(pud_t *pud, unsigned long addr, struct hstate *hstate)
+{
+ if (huge_page_shift(hstate) < PMD_SHIFT)
return pmd_offset(pud, addr);
else
return (pmd_t *) pud;
}
-static inline
-pmd_t *hpmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long addr,
- struct hstate *hstate)
+static pmd_t *hpmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long addr,
+ struct hstate *hstate)
{
- if (huge_page_shift(hstate) == PAGE_SHIFT_64K)
+ if (huge_page_shift(hstate) < PMD_SHIFT)
return pmd_alloc(mm, pud, addr);
else
return (pmd_t *) pud;
}
-#endif

/* Build list of addresses of gigantic pages. This function is used in early
* boot before the buddy or bootmem allocator is setup.
@@ -204,7 +212,7 @@ pte_t *huge_pte_offset(struct mm_struct

pg = pgd_offset(mm, addr);
if (!pgd_none(*pg)) {
- pu = pud_offset(pg, addr);
+ pu = hpud_offset(pg, addr, hstate);
if (!pud_none(*pu)) {
pm = hpmd_offset(pu, addr, hstate);
if (!pmd_none(*pm))
@@ -233,7 +241,7 @@ pte_t *huge_pte_alloc(struct mm_struct *
addr &= hstate->mask;

pg = pgd_offset(mm, addr);
- pu = pud_alloc(mm, pg, addr);
+ pu = hpud_alloc(mm, pg, addr, hstate);

if (pu) {
pm = hpmd_alloc(mm, pu, addr, hstate);
@@ -316,13 +324,7 @@ static void hugetlb_free_pud_range(struc
pud = pud_offset(pgd, addr);
do {
next = pud_addr_end(addr, end);
-#ifdef CONFIG_PPC_64K_PAGES
- if (pud_none_or_clear_bad(pud))
- continue;
- hugetlb_free_pmd_range(tlb, pud, addr, next, floor, ceiling,
- psize);
-#else
- if (shift == PAGE_SHIFT_64K) {
+ if (shift < PMD_SHIFT) {
if (pud_none_or_clear_bad(pud))
continue;
hugetlb_free_pmd_range(tlb, pud, addr, next, floor,
@@ -332,7 +334,6 @@ static void hugetlb_free_pud_range(struc
continue;
free_hugepte_range(tlb, (hugepd_t *)pud, psize);
}
-#endif
} while (pud++, addr = next, addr != end);

start &= PGDIR_MASK;
@@ -422,9 +423,15 @@ void hugetlb_free_pgd_range(struct mmu_g
psize = get_slice_psize(tlb->mm, addr);
BUG_ON(!mmu_huge_psizes[psize]);
next = pgd_addr_end(addr, end);
- if (pgd_none_or_clear_bad(pgd))
- continue;
- hugetlb_free_pud_range(tlb, pgd, addr, next, floor, ceiling);
+ if (mmu_psize_to_shift(psize) < PUD_SHIFT) {
+ if (pgd_none_or_clear_bad(pgd))
+ continue;
+ hugetlb_free_pud_range(tlb, pgd, addr, next, floor, ceiling);
+ } else {
+ if (pgd_none(*pgd))
+ continue;
+ free_hugepte_range(tlb, (hugepd_t *)pgd, psize);
+ }
} while (pgd++, addr = next, addr != end);
}

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!