[PATCH v5 2/8] mm/hugetlb: pmd_huge() returns true for non-present hugepage

From: Naoya Horiguchi
Date: Tue Dec 02 2014 - 03:30:31 EST

Migrating hugepages and hwpoisoned hugepages are considered as
non-present hugepages, and they are referenced via migration entries
and hwpoison entries in their page table slots.

This behavior causes race condition because pmd_huge() doesn't tell
non-huge pages from migrating/hwpoisoned hugepages. follow_page_mask()
is one example where the kernel would call follow_page_pte() for such
hugepage while this function is supposed to handle only normal pages.

To avoid this, this patch makes pmd_huge() return true when pmd_none()
is true *and* pmd_present() is false. We don't have to worry about mixing
up non-present pmd entry with normal pmd (pointing to leaf level pte
entry) because pmd_present() is true in normal pmd.

The same race condition could happen in (x86-specific) gup_pmd_range(),
where this patch simply adds pmd_present() check instead of pmd_huge().
This is because gup_pmd_range() is fast path. If we have non-present
hugepage in this function, we will go into gup_huge_pmd(), then return
0 at flag mask check, and finally fall back to the slow path.

Fixes: 290408d4a2 ("hugetlb: hugepage migration core")
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # [2.6.36+]
arch/x86/mm/gup.c | 2 +-
arch/x86/mm/hugetlbpage.c | 8 +++++++-
mm/hugetlb.c | 2 ++
3 files changed, 10 insertions(+), 2 deletions(-)

diff --git mmotm-2014-11-26-15-45.orig/arch/x86/mm/gup.c mmotm-2014-11-26-15-45/arch/x86/mm/gup.c
index 207d9aef662d..448ee8912d9b 100644
--- mmotm-2014-11-26-15-45.orig/arch/x86/mm/gup.c
+++ mmotm-2014-11-26-15-45/arch/x86/mm/gup.c
@@ -172,7 +172,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
if (pmd_none(pmd) || pmd_trans_splitting(pmd))
return 0;
- if (unlikely(pmd_large(pmd))) {
+ if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
* NUMA hinting faults need to be handled in the GUP
* slowpath for accounting purposes and so that they
diff --git mmotm-2014-11-26-15-45.orig/arch/x86/mm/hugetlbpage.c mmotm-2014-11-26-15-45/arch/x86/mm/hugetlbpage.c
index 03b8a7c11817..9161f764121e 100644
--- mmotm-2014-11-26-15-45.orig/arch/x86/mm/hugetlbpage.c
+++ mmotm-2014-11-26-15-45/arch/x86/mm/hugetlbpage.c
@@ -54,9 +54,15 @@ int pud_huge(pud_t pud)


+ * pmd_huge() returns 1 if @pmd is hugetlb related entry, that is normal
+ * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
+ * Otherwise, returns 0.
+ */
int pmd_huge(pmd_t pmd)
- return !!(pmd_val(pmd) & _PAGE_PSE);
+ return !pmd_none(pmd) &&
+ (pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;

int pud_huge(pud_t pud)
diff --git mmotm-2014-11-26-15-45.orig/mm/hugetlb.c mmotm-2014-11-26-15-45/mm/hugetlb.c
index 6be4a690e554..dd42878549d5 100644
--- mmotm-2014-11-26-15-45.orig/mm/hugetlb.c
+++ mmotm-2014-11-26-15-45/mm/hugetlb.c
@@ -3679,6 +3679,8 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
struct page *page;

+ if (!pmd_present(*pmd))
+ return NULL;
page = pte_page(*(pte_t *)pmd);
if (page)
page += ((address & ~PMD_MASK) >> PAGE_SHIFT);
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