[PATCH 3.19 031/177] mm/hugetlb: fix getting refcount 0 page in hugetlb_fault()

Greg Kroah-Hartman
2015-03-16T15:59:34Z

From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

commit 0f792cf949a0be506c2aa8bfac0605746b146dda upstream.

When running the test which causes the race as shown in the previous patch,
we can hit the BUG "get_page() on refcount 0 page" in hugetlb_fault().

This race happens when pte turns into migration entry just after the first
check of is_hugetlb_entry_migration() in hugetlb_fault() passed with false.
To fix this, we need to check pte_present() again after huge_ptep_get().

This patch also reorders taking ptl and doing pte_page(), because
pte_page() should be done in ptl. Due to this reordering, we need use
trylock_page() in page != pagecache_page case to respect locking order.

Fixes: 66aebce747ea ("hugetlb: fix race condition in hugetlb_fault()")
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/hugetlb.c | 52 ++++++++++++++++++++++++++++++++++++----------------
1 file changed, 36 insertions(+), 16 deletions(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3134,6 +3134,7 @@ int hugetlb_fault(struct mm_struct *mm,
struct page *pagecache_page = NULL;
struct hstate *h = hstate_vma(vma);
struct address_space *mapping;
+ int need_wait_lock = 0;

address &= huge_page_mask(h);

@@ -3172,6 +3173,16 @@ int hugetlb_fault(struct mm_struct *mm,
ret = 0;

/*
+ * entry could be a migration/hwpoison entry at this point, so this
+ * check prevents the kernel from going below assuming that we have
+ * a active hugepage in pagecache. This goto expects the 2nd page fault,
+ * and is_hugetlb_entry_(migration|hwpoisoned) check will properly
+ * handle it.
+ */
+ if (!pte_present(entry))
+ goto out_mutex;
+
+ /*
* If we are going to COW the mapping later, we examine the pending
* reservations for this page now. This will ensure that any
* allocations necessary to record that reservation occur outside the
@@ -3190,30 +3201,31 @@ int hugetlb_fault(struct mm_struct *mm,
vma, address);
}

+ ptl = huge_pte_lock(h, mm, ptep);
+
+ /* Check for a racing update before calling hugetlb_cow */
+ if (unlikely(!pte_same(entry, huge_ptep_get(ptep))))
+ goto out_ptl;
+
/*
* hugetlb_cow() requires page locks of pte_page(entry) and
* pagecache_page, so here we need take the former one
* when page != pagecache_page or !pagecache_page.
- * Note that locking order is always pagecache_page -> page,
- * so no worry about deadlock.
*/
page = pte_page(entry);
- get_page(page);
if (page != pagecache_page)
- lock_page(page);
-
- ptl = huge_pte_lockptr(h, mm, ptep);
- spin_lock(ptl);
- /* Check for a racing update before calling hugetlb_cow */
- if (unlikely(!pte_same(entry, huge_ptep_get(ptep))))
- goto out_ptl;
+ if (!trylock_page(page)) {
+ need_wait_lock = 1;
+ goto out_ptl;
+ }

+ get_page(page);

if (flags & FAULT_FLAG_WRITE) {
if (!huge_pte_write(entry)) {
ret = hugetlb_cow(mm, vma, address, ptep, entry,
pagecache_page, ptl);
- goto out_ptl;
+ goto out_put_page;
}
entry = huge_pte_mkdirty(entry);
}
@@ -3221,7 +3233,10 @@ int hugetlb_fault(struct mm_struct *mm,
if (huge_ptep_set_access_flags(vma, address, ptep, entry,
flags & FAULT_FLAG_WRITE))
update_mmu_cache(vma, address, ptep);
-
+out_put_page:
+ if (page != pagecache_page)
+ unlock_page(page);
+ put_page(page);
out_ptl:
spin_unlock(ptl);

@@ -3229,12 +3244,17 @@ out_ptl:
unlock_page(pagecache_page);
put_page(pagecache_page);
}
- if (page != pagecache_page)
- unlock_page(page);
- put_page(page);
-
out_mutex:
mutex_unlock(&htlb_fault_mutex_table[hash]);
+ /*
+ * Generally it's safe to hold refcount during waiting page lock. But
+ * here we just wait to defer the next page fault to avoid busy loop and
+ * the page is not used after unlocked before returning from the current
+ * page fault. So we are safe from accessing freed page, even if we wait
+ * here without taking refcount.
+ */
+ if (need_wait_lock)
+ wait_on_page_locked(page);
return ret;
}