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mm/hugetlb: fix getting refcount 0 page in hugetlb_fault()

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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: 66aebce747 ("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>
Cc: <stable@vger.kernel.org>	[3.2+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Naoya Horiguchi 2015-02-11 15:25:25 -08:00 committed by Linus Torvalds
parent e66f17ff71
commit 0f792cf949
1 changed files with 39 additions and 19 deletions
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58
mm/hugetlb.c
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@ -3134,6 +3134,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
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);
@ -3171,6 +3172,16 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
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
@ -3190,30 +3201,31 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
vma, address);
}
/*
* 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_lock(h, mm, ptep);
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;
/*
* 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.
*/
page = pte_page(entry);
if (page != pagecache_page)
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, struct vm_area_struct *vma,
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;
}