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Subject[07/24] tmpfs: fix race between umount and swapoff
2.6.33-longterm review patch.  If anyone has any objections, please let us know.

------------------
Content-Length: 6159
Lines: 197
From: Hugh Dickins <hughd@google.com>

commit 778dd893ae785c5fd505dac30b5fc40aae188bf1 upstream.

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>
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@suse.de>

---
mm/shmem.c | 88 +++++++++++++++++++++++++++++--------------------------------
1 file changed, 43 insertions(+), 45 deletions(-)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -850,7 +850,7 @@ static inline int shmem_find_swp(swp_ent

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;
@@ -873,8 +873,10 @@ static int shmem_unuse_inode(struct shme
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;

@@ -912,11 +914,11 @@ static int shmem_unuse_inode(struct shme
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:
@@ -926,8 +928,7 @@ 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.
@@ -938,37 +939,18 @@ 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) {
/*
@@ -988,14 +970,8 @@ 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;
}

@@ -1007,6 +983,26 @@ int shmem_unuse(swp_entry_t entry, struc
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) {
@@ -1014,17 +1010,19 @@ int shmem_unuse(swp_entry_t entry, struc
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;
}

/*



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