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

From: David Gibson <[email-masked]>
Date: Wed, 3 Sep 2008 10:20:29 +1000

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_!
http://www.ozlabs.org/~dgibson