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-horigu...@ah.jp.nec.com>
Cc: <sta...@vger.kernel.org> # [3.2+]
---
ChangeLog v4:
- move !pte_present(entry) (i.e. migration/hwpoison) check before
taking page table lock
- call wait_on_page_locked() if trylock_page() returns false
- remove unused label out_unlock_page
- fix the order of put_page() and unlock_page() after out_put_page label
- move changelog under '---'
Hugh advised me for ver.3 that we can call migration_entry_wait_huge()
when the !pte_present(entry) check returns true to avoid busy faulting.
But it seems that in that case only one additional page fault happens
instead of busy faulting, because is_hugetlb_entry_migration() in the
second call of hugetlb_fault() should return true and then
migration_entry_wait_huge() is called. We could avoid this additional
page fault by adding another migration_entry_wait_huge(), but then
we should separate pte_present() check into is_hugetlb_entry_migration()
path and is_hugetlb_entry_hwpoisoned() path, which makes code complicated.
So let me take the simpler approach for sending stable tree.
And it's also advised that we can clean up is_hugetlb_entry_migration()
and is_hugetlb_entry_hwpoisoned() things. This will be done in another
work, and the above migration_entry_wait_huge problem will be revisited
there.
ChangeLog v3:
- doing pte_page() and taking refcount under page table lock
- check pte_present after taking ptl, which makes it unnecessary to use
get_page_unless_zero()
- use trylock_page in page != pagecache_page case
- fixed target stable version
---
mm/hugetlb.c | 42 ++++++++++++++++++++++++++++--------------
1 file changed, 28 insertions(+), 14 deletions(-)
diff --git mmotm-2014-09-09-14-42.orig/mm/hugetlb.c
mmotm-2014-09-09-14-42/mm/hugetlb.c
index 941832ee3d5a..52f001943169 100644
--- mmotm-2014-09-09-14-42.orig/mm/hugetlb.c
+++ mmotm-2014-09-09-14-42/mm/hugetlb.c
@@ -3128,6 +3128,7 @@ 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);
@@ -3164,6 +3165,15 @@ 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
@@ -3184,6 +3194,12 @@ int hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
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
@@ -3192,22 +3208,19 @@ int hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
* 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);
}
@@ -3215,7 +3228,10 @@ 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);
@@ -3223,12 +3239,10 @@ int hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
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]);
+ if (need_wait_lock)
+ wait_on_page_locked(page);
return ret;
}
--
1.9.3
--
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