tmpfs: fix race between umount and swapoff

The use of igrab() in swapoff's shmem_unuse_inode() is just as vulnerable
to umount as that in shmem_writepage().

Fix this instance by extending the protection of shmem_swaplist_mutex
right across shmem_unuse_inode(): while it's on the list, the inode cannot
be evicted (and the filesystem cannot be unmounted) without
shmem_evict_inode() taking that mutex to remove it from the list.

But since shmem_writepage() might take that mutex, we should avoid making
memory allocations or memcg charges while holding it: prepare them at the
outer level in shmem_unuse().  When mem_cgroup_cache_charge() was
originally placed, we didn't know until that point that the page from swap
was actually a shmem page; but nowadays it's noted in the swap_map, so
we're safe to charge upfront.  For the radix_tree, do as is done in
shmem_getpage(): preload upfront, but don't pin to the cpu; so we make a
habit of refreshing the node pool, but might dip into GFP_NOWAIT reserves
on occasion if subsequently preempted.

With the allocation and charge moved out from shmem_unuse_inode(),
we can also hold index map and info->lock over from finding the entry.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 43 insertions, 45 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 262d71173447..dc17551d060a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -852,7 +852,7 @@ static inline int shmem_find_swp(swp_entry_t entry, swp_entry_t *dir, swp_entry_
 
 static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, struct page *page)
 {
-        struct inode *inode;
+        struct address_space *mapping;
         unsigned long idx;
         unsigned long size;
         unsigned long limit;
@@ -875,8 +875,10 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
         if (size > SHMEM_NR_DIRECT)
                 size = SHMEM_NR_DIRECT;
         offset = shmem_find_swp(entry, ptr, ptr+size);
-        if (offset >= 0)
+        if (offset >= 0) {
+                shmem_swp_balance_unmap();
                 goto found;
+        }
         if (!info->i_indirect)
                 goto lost2;
 
@@ -914,11 +916,11 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
                         if (size > ENTRIES_PER_PAGE)
                                 size = ENTRIES_PER_PAGE;
                         offset = shmem_find_swp(entry, ptr, ptr+size);
-                        shmem_swp_unmap(ptr);
                         if (offset >= 0) {
                                 shmem_dir_unmap(dir);
                                 goto found;
                         }
+                        shmem_swp_unmap(ptr);
                 }
         }
 lost1:
@@ -928,8 +930,7 @@ lost2:
         return 0;
 found:
         idx += offset;
-        inode = igrab(&info->vfs_inode);
-        spin_unlock(&info->lock);
+        ptr += offset;
 
         /*
          * Move _head_ to start search for next from here.
@@ -940,37 +941,18 @@ found:
          */
         if (shmem_swaplist.next != &info->swaplist)
                 list_move_tail(&shmem_swaplist, &info->swaplist);
-        mutex_unlock(&shmem_swaplist_mutex);
 
-        error = 1;
-        if (!inode)
-                goto out;
         /*
-         * Charge page using GFP_KERNEL while we can wait.
-         * Charged back to the user(not to caller) when swap account is used.
-         * add_to_page_cache() will be called with GFP_NOWAIT.
+         * We rely on shmem_swaplist_mutex, not only to protect the swaplist,
+         * but also to hold up shmem_evict_inode(): so inode cannot be freed
+         * beneath us (pagelock doesn't help until the page is in pagecache).
          */
-        error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);
-        if (error)
-                goto out;
-        error = radix_tree_preload(GFP_KERNEL);
-        if (error) {
-                mem_cgroup_uncharge_cache_page(page);
-                goto out;
-        }
-        error = 1;
-
-        spin_lock(&info->lock);
-        ptr = shmem_swp_entry(info, idx, NULL);
-        if (ptr && ptr->val == entry.val) {
-                error = add_to_page_cache_locked(page, inode->i_mapping,
-                                                idx, GFP_NOWAIT);
-                /* does mem_cgroup_uncharge_cache_page on error */
-        } else  /* we must compensate for our precharge above */
-                mem_cgroup_uncharge_cache_page(page);
+        mapping = info->vfs_inode.i_mapping;
+        error = add_to_page_cache_locked(page, mapping, idx, GFP_NOWAIT);
+        /* which does mem_cgroup_uncharge_cache_page on error */
 
         if (error == -EEXIST) {
-                struct page *filepage = find_get_page(inode->i_mapping, idx);
+                struct page *filepage = find_get_page(mapping, idx);
                 error = 1;
                 if (filepage) {
                         /*
@@ -990,14 +972,8 @@ found:
                 swap_free(entry);
                 error = 1;      /* not an error, but entry was found */
         }
-        if (ptr)
-                shmem_swp_unmap(ptr);
+        shmem_swp_unmap(ptr);
         spin_unlock(&info->lock);
-        radix_tree_preload_end();
-out:
-        unlock_page(page);
-        page_cache_release(page);
-        iput(inode);            /* allows for NULL */
         return error;
 }
 
@@ -1009,6 +985,26 @@ int shmem_unuse(swp_entry_t entry, struct page *page)
         struct list_head *p, *next;
         struct shmem_inode_info *info;
         int found = 0;
+        int error;
+
+        /*
+         * Charge page using GFP_KERNEL while we can wait, before taking
+         * the shmem_swaplist_mutex which might hold up shmem_writepage().
+         * Charged back to the user (not to caller) when swap account is used.
+         * add_to_page_cache() will be called with GFP_NOWAIT.
+         */
+        error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);
+        if (error)
+                goto out;
+        /*
+         * Try to preload while we can wait, to not make a habit of
+         * draining atomic reserves; but don't latch on to this cpu,
+         * it's okay if sometimes we get rescheduled after this.
+         */
+        error = radix_tree_preload(GFP_KERNEL);
+        if (error)
+                goto uncharge;
+        radix_tree_preload_end();
 
         mutex_lock(&shmem_swaplist_mutex);
         list_for_each_safe(p, next, &shmem_swaplist) {
@@ -1016,17 +1012,19 @@ int shmem_unuse(swp_entry_t entry, struct page *page)
                 found = shmem_unuse_inode(info, entry, page);
                 cond_resched();
                 if (found)
-                        goto out;
+                        break;
         }
         mutex_unlock(&shmem_swaplist_mutex);
-        /*
-         * Can some race bring us here?  We've been holding page lock,
-         * so I think not; but would rather try again later than BUG()
-         */
+
+uncharge:
+        if (!found)
+                mem_cgroup_uncharge_cache_page(page);
+        if (found < 0)
+                error = found;
+out:
         unlock_page(page);
         page_cache_release(page);
-out:
-        return (found < 0) ? found : 0;
+        return error;
 }
 
 /*