On Fri, Jan 04, 2013 at 05:03:26AM +0000, Christoffer Dall wrote:

On Thu, Oct 18, 2012 at 12:15 PM, Steve Capper [off-list ref] wrote:

quoted

 /*
+ * For 3 levels of paging the PTE_EXT_NG bit will be set for user address ptes
+ * that are written to a page table but not for ptes created with mk_pte.
+ *

Why is this not the case for 2 levels of paging as well?

Is that because it's always checked against the Linux version, or?

Yes that's the case, I'll update the comment to reflect that.

quoted

+ * This can cause some comparison tests made by pte_same to fail spuriously and
+ * lead to other problems.
+ *
+ * To correct this behaviour, we mask off PTE_EXT_NG for any pte that is
+ * present before running the comparison.

nit: This comment doesn't really explain the rationale, I'm assuming
that pte_same is used to compare only which page gets mapped, assuming
the attributes etc. remain the same? or also the attributes should be
the same, only mk_pte sets all of these except the NG bit.

I'll expand the comment to include the actual case. Essentially hugetlb_nopage
calls mk_pte to give new_pte and passes this to hugetlb_cow which then performs
a pte_same test against a pte that has already been written out to a page
table; the test fails erroneously due to the mismatch in NG bit.

Unfortunately this then causes a memory leak. 

quoted

+ */
+#define __HAVE_ARCH_PTE_SAME
+static inline int pte_same(pte_t pte_a, pte_t pte_b)
+{
+       pteval_t vala = pte_val(pte_a), valb = pte_val(pte_b);
+       if (pte_present(pte_a))
+               vala &= ~L_PTE_CMP_MASKOFF;
+
+       if (pte_present(pte_b))
+               valb &= ~L_PTE_CMP_MASKOFF;
+
+       return vala == valb;
+}
+
+/*
  * Encode and decode a swap entry.  Swap entries are stored in the Linux
  * page tables as follows:
  *
--
1.7.9.5



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