tmpfs: fix shared mempolicy leak

This fixes a regression in 3.7-rc, which has since gone into stable.

Commit 00442ad04a5e ("mempolicy: fix a memory corruption by refcount
imbalance in alloc_pages_vma()") changed get_vma_policy() to raise the
refcount on a shmem shared mempolicy; whereas shmem_alloc_page() went
on expecting alloc_page_vma() to drop the refcount it had acquired.
This deserves a rework: but for now fix the leak in shmem_alloc_page().

Hugh: shmem_swapin() did not need a fix, but surely it's clearer to use
the same refcounting there as in shmem_alloc_page(), delete its onstack
mempolicy, and the strange mpol_cond_copy() and __mpol_cond_copy() -
those were invented to let swapin_readahead() make an unknown number of
calls to alloc_pages_vma() with one mempolicy; but since 00442ad04a5e,
alloc_pages_vma() has kept refcount in balance, so now no problem.

Reported-and-tested-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 16 insertions, 48 deletions

diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index e5ccb9ddd90e..dbd212723b74 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -82,16 +82,6 @@ static inline void mpol_cond_put(struct mempolicy *pol)
                 __mpol_put(pol);
 }
 
-extern struct mempolicy *__mpol_cond_copy(struct mempolicy *tompol,
-                                          struct mempolicy *frompol);
-static inline struct mempolicy *mpol_cond_copy(struct mempolicy *tompol,
-                                                struct mempolicy *frompol)
-{
-        if (!frompol)
-                return frompol;
-        return __mpol_cond_copy(tompol, frompol);
-}
-
 extern struct mempolicy *__mpol_dup(struct mempolicy *pol);
 static inline struct mempolicy *mpol_dup(struct mempolicy *pol)
 {
@@ -215,12 +205,6 @@ static inline void mpol_cond_put(struct mempolicy *pol)
 {
 }
 
-static inline struct mempolicy *mpol_cond_copy(struct mempolicy *to,
-                                                struct mempolicy *from)
-{
-        return from;
-}
-
 static inline void mpol_get(struct mempolicy *pol)
 {
 }
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index d04a8a54c294..4ea600da8940 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2037,28 +2037,6 @@ struct mempolicy *__mpol_dup(struct mempolicy *old)
         return new;
 }
 
-/*
- * If *frompol needs [has] an extra ref, copy *frompol to *tompol ,
- * eliminate the * MPOL_F_* flags that require conditional ref and
- * [NOTE!!!] drop the extra ref.  Not safe to reference *frompol directly
- * after return.  Use the returned value.
- *
- * Allows use of a mempolicy for, e.g., multiple allocations with a single
- * policy lookup, even if the policy needs/has extra ref on lookup.
- * shmem_readahead needs this.
- */
-struct mempolicy *__mpol_cond_copy(struct mempolicy *tompol,
-                                                struct mempolicy *frompol)
-{
-        if (!mpol_needs_cond_ref(frompol))
-                return frompol;
-
-        *tompol = *frompol;
-        tompol->flags &= ~MPOL_F_SHARED;        /* copy doesn't need unref */
-        __mpol_put(frompol);
-        return tompol;
-}
-
 /* Slow path of a mempolicy comparison */
 bool __mpol_equal(struct mempolicy *a, struct mempolicy *b)
 {
diff --git a/mm/shmem.c b/mm/shmem.c
index 89341b658bd0..50c5b8f3a359 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -910,25 +910,29 @@ static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
 static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
                         struct shmem_inode_info *info, pgoff_t index)
 {
-        struct mempolicy mpol, *spol;
         struct vm_area_struct pvma;
-
-        spol = mpol_cond_copy(&mpol,
-                        mpol_shared_policy_lookup(&info->policy, index));
+        struct page *page;
 
         /* Create a pseudo vma that just contains the policy */
         pvma.vm_start = 0;
         /* Bias interleave by inode number to distribute better across nodes */
         pvma.vm_pgoff = index + info->vfs_inode.i_ino;
         pvma.vm_ops = NULL;
-        pvma.vm_policy = spol;
-        return swapin_readahead(swap, gfp, &pvma, 0);
+        pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, index);
+
+        page = swapin_readahead(swap, gfp, &pvma, 0);
+
+        /* Drop reference taken by mpol_shared_policy_lookup() */
+        mpol_cond_put(pvma.vm_policy);
+
+        return page;
 }
 
 static struct page *shmem_alloc_page(gfp_t gfp,
                         struct shmem_inode_info *info, pgoff_t index)
 {
         struct vm_area_struct pvma;
+        struct page *page;
 
         /* Create a pseudo vma that just contains the policy */
         pvma.vm_start = 0;
@@ -937,10 +941,12 @@ static struct page *shmem_alloc_page(gfp_t gfp,
         pvma.vm_ops = NULL;
         pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, index);
 
-        /*
-         * alloc_page_vma() will drop the shared policy reference
-         */
-        return alloc_page_vma(gfp, &pvma, 0);
+        page = alloc_page_vma(gfp, &pvma, 0);
+
+        /* Drop reference taken by mpol_shared_policy_lookup() */
+        mpol_cond_put(pvma.vm_policy);
+
+        return page;
 }
 #else /* !CONFIG_NUMA */
 #ifdef CONFIG_TMPFS