[PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries

Will Deacon [off-list ref] writes:

On Wed, Jun 07, 2017 at 04:32:28PM +0100, Punit Agrawal wrote:

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Will Deacon [off-list ref] writes:

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On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:

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On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:

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On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:

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--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c

@@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
 {
 	pgd_t *pgd;
 	pud_t *pud;
-	pmd_t *pmd = NULL;
-	pte_t *pte = NULL;
+	pmd_t *pmd;
 
 	pgd = pgd_offset(mm, addr);
 	pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
 	if (!pgd_present(*pgd))
 		return NULL;
+
 	pud = pud_offset(pgd, addr);
-	if (!pud_present(*pud))
+	if (pud_none(*pud))
 		return NULL;

Do you actually need this special case?

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-
-	if (pud_huge(*pud))
+	/* swap or huge page */
+	if (!pud_present(*pud) || pud_huge(*pud))

... couldn't you just add a '|| pud_none(*pud)' in here?

I think an earlier version took this approach but...

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 		return (pte_t *)pud;

But then you no longer return NULL if *pud == 0.

Does that actually matter? The bits of hugetlb code I looked at will
deferenced the returned pud and handle the huge_pte_none case correctly.

For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
to the pud/pmd results in different behaviour. If we return the pud when
pud_none(), then we lose the resulting hugepage size check we get from
huge_pte_alloc().

Ok, so does that mean that many of the huge_pte_none checks in mm/hugetlb.c
that operate on a huge_ptep_get of non-NULL output from huge_pte_offset are
actually redundant?

Summarising our offline discussion - the semantics of huge_pte_offset()
are unclear in terms of when a pointer to (p*d) is returned vs when to
return NULL.

As part of enabling contiguous hugepage support[0][1], I have a patch to
add a size argument to huge_pte_offset() that can then help disambiguate
when we have a valid pte* vs when we have an error (NULL).

I'll separately look at clarifying the semantics of the generic version
of huge_pte_offse() and potentially also look at unifying the
huge_pte_offset() implementations across the various architectures.

[0] https://lkml.org/lkml/2017/5/24/463
[1] https://www.spinics.net/lists/arm-kernel/msg583367.html

Will