tmpfs: fix shmem_swaplist races

Intensive swapoff testing shows shmem_unuse spinning on an entry in
shmem_swaplist pointing to itself: how does that come about?  Days pass...

First guess is this: shmem_delete_inode tests list_empty without taking the
global mutex (so the swapping case doesn't slow down the common case); but
there's an instant in shmem_unuse_inode's list_move_tail when the list entry
may appear empty (a rare case, because it's actually moving the head not the
the list member).  So there's a danger of leaving the inode on the swaplist
when it's freed, then reinitialized to point to itself when reused.  Fix that
by skipping the list_move_tail when it's a no-op, which happens to plug this.

But this same spinning then surfaces on another machine.  Ah, I'd never
suspected it, but shmem_writepage's swaplist manipulation is unsafe: though we
still hold page lock, which would hold off inode deletion if the page were in
pagecache, it doesn't hold off once it's in swapcache (free_swap_and_cache
doesn't wait on locked pages).  Hmm: we could put the the inode on swaplist
earlier, but then shmem_unuse_inode could never prune unswapped inodes.

Fix this with an igrab before dropping info->lock, as in shmem_unuse_inode;
though I am a little uneasy about the iput which has to follow - it works, and
I see nothing wrong with it, but it is surprising that shmem inode deletion
may now occur below shmem_writepage.  Revisit this fix later?

And while we're looking at these races: the way shmem_unuse tests swapped
without holding info->lock looks unsafe, if we've more than one swap area: a
racing shmem_writepage on another page of the same inode could be putting it
in swapcache, just as we're deciding to remove the inode from swaplist -
there's a danger of going on swap without being listed, so a later swapoff
would hang, being unable to locate the entry.  Move that test and removal down
into shmem_unuse_inode, once info->lock is held.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 25 insertions, 12 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 530c5033d028..ee9024483f60 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -833,6 +833,10 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
         idx = 0;
         ptr = info->i_direct;
         spin_lock(&info->lock);
+        if (!info->swapped) {
+                list_del_init(&info->swaplist);
+                goto lost2;
+        }
         limit = info->next_index;
         size = limit;
         if (size > SHMEM_NR_DIRECT)
@@ -894,8 +898,15 @@ found:
         inode = igrab(&info->vfs_inode);
         spin_unlock(&info->lock);
 
-        /* move head to start search for next from here */
-        list_move_tail(&shmem_swaplist, &info->swaplist);
+        /*
+         * Move _head_ to start search for next from here.
+         * But be careful: shmem_delete_inode checks list_empty without taking
+         * mutex, and there's an instant in list_move_tail when info->swaplist
+         * would appear empty, if it were the only one on shmem_swaplist.  We
+         * could avoid doing it if inode NULL; or use this minor optimization.
+         */
+        if (shmem_swaplist.next != &info->swaplist)
+                list_move_tail(&shmem_swaplist, &info->swaplist);
         mutex_unlock(&shmem_swaplist_mutex);
 
         error = 1;
@@ -955,10 +966,7 @@ int shmem_unuse(swp_entry_t entry, struct page *page)
         mutex_lock(&shmem_swaplist_mutex);
         list_for_each_safe(p, next, &shmem_swaplist) {
                 info = list_entry(p, struct shmem_inode_info, swaplist);
-                if (info->swapped)
-                        found = shmem_unuse_inode(info, entry, page);
-                else
-                        list_del_init(&info->swaplist);
+                found = shmem_unuse_inode(info, entry, page);
                 cond_resched();
                 if (found)
                         goto out;
@@ -1021,18 +1029,23 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
                 remove_from_page_cache(page);
                 shmem_swp_set(info, entry, swap.val);
                 shmem_swp_unmap(entry);
+                if (list_empty(&info->swaplist))
+                        inode = igrab(inode);
+                else
+                        inode = NULL;
                 spin_unlock(&info->lock);
-                if (list_empty(&info->swaplist)) {
-                        mutex_lock(&shmem_swaplist_mutex);
-                        /* move instead of add in case we're racing */
-                        list_move_tail(&info->swaplist, &shmem_swaplist);
-                        mutex_unlock(&shmem_swaplist_mutex);
-                }
                 swap_duplicate(swap);
                 BUG_ON(page_mapped(page));
                 page_cache_release(page);       /* pagecache ref */
                 set_page_dirty(page);
                 unlock_page(page);
+                if (inode) {
+                        mutex_lock(&shmem_swaplist_mutex);
+                        /* move instead of add in case we're racing */
+                        list_move_tail(&info->swaplist, &shmem_swaplist);
+                        mutex_unlock(&shmem_swaplist_mutex);
+                        iput(inode);
+                }
                 return 0;
         }