[RESEND][PATCH v5 3/3] hugetlbfs: add swap entry check in follow_hugetlb_page()

From: Naoya Horiguchi
Date: Wed Apr 10 2013 - 12:19:55 EST

# I suspended Reviewed and Acked given for the previous version, because
# it has a non-minor change. If you want to restore it, please let me know.
With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
error happens on a hugepage and the affected processes try to access
the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
in get_page().

The reason for this bug is that coredump-related code doesn't recognise
"hugepage hwpoison entry" with which a pmd entry is replaced when a memory
error occurs on a hugepage.
In other words, physical address information is stored in different bit layout
between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
which is called in get_dump_page() returns a wrong page from a given address.

The expected behavior is like this:

absent is_swap_pte FOLL_DUMP Expected behavior
true false false hugetlb_fault
false true false hugetlb_fault
false false false return page
true false true skip page (to avoid allocation)
false true true hugetlb_fault
false false true return page

With this patch, we can call hugetlb_fault() and take proper actions
(we wait for migration entries, fail with VM_FAULT_HWPOISON_LARGE for
hwpoisoned entries,) and as the result we can dump all hugepages except
for hwpoisoned ones.

ChangeLog v5:
- improve comment and description.

ChangeLog v4:
- move is_swap_page() to right place.

ChangeLog v3:
- add comment about using is_swap_pte()

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
mm/hugetlb.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
index 0d1705b..bf26ee8 100644
--- v3.9-rc3.orig/mm/hugetlb.c
+++ v3.9-rc3/mm/hugetlb.c
@@ -2983,7 +2983,17 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,

- if (absent ||
+ /*
+ * We need call hugetlb_fault for both hugepages under migration
+ * (in which case hugetlb_fault waits for the migration,) and
+ * hwpoisoned hugepages (in which case we need to prevent the
+ * caller from accessing to them.) In order to do this, we use
+ * here is_swap_pte instead of is_hugetlb_entry_migration and
+ * is_hugetlb_entry_hwpoisoned. This is because it simply covers
+ * both cases, and because we can't follow correct pages
+ * directly from any kind of swap entries.
+ */
+ if (absent || is_swap_pte(huge_ptep_get(pte)) ||
((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
int ret;


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