In many places !pmd_present has been converted to pmd_none. For pmds
that's equivalent and pmd_none is quicker so using pmd_none is
better.
However (unless we delete pmd_present) we should provide an accurate
pmd_present too. This will avoid the risk of code thinking the pmd is
non present because it's under __split_huge_page_map, see the
pmd_mknotpresent there and the comment above it.
If the page has been mprotected as PROT_NONE, it would also lead to a
pmd_present false negative in the same way as the race with
split_huge_page.
Because the PSE bit stays on at all times (both during split_huge_page
and when the _PAGE_PROTNONE bit get set), we could only check for the
PSE bit, but checking the PROTNONE bit too is still good to remember
pmd_present must always keep PROT_NONE into account.
This explains a not reproducible BUG_ON that was seldom reported on
the lists.
The same issue is in pmd_large, it would go wrong with both PROT_NONE
and if it races with split_huge_page.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
arch/x86/include/asm/pgtable.h | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 49afb3f..c3520d7 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -146,8 +146,7 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
static inline int pmd_large(pmd_t pte)
{
- return (pmd_flags(pte) & (_PAGE_PSE | _PAGE_PRESENT)) ==
- (_PAGE_PSE | _PAGE_PRESENT);
+ return pmd_flags(pte) & _PAGE_PSE;
}
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
@@ -415,7 +414,13 @@ static inline int pte_hidden(pte_t pte)
static inline int pmd_present(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_PRESENT;
+ /*
+ * Checking for _PAGE_PSE is needed too because
+ * split_huge_page will temporarily clear the present bit (but
+ * the _PAGE_PSE flag will remain set at all times while the
+ * _PAGE_PRESENT bit is clear).
+ */
+ return pmd_flags(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE);
}
static inline int pmd_none(pmd_t pmd)
--
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