ext4: fix BUG_ON in mb_free_blocks()

commit c99d1e6e83b06744c75d9f5e491ed495a7086b7b upstream.

If we suffer a block allocation failure (for example due to a memory
allocation failure), it's possible that we will call
ext4_discard_allocated_blocks() before we've actually allocated any
blocks.  In that case, fe_len and fe_start in ac->ac_f_ex will still
be zero, and this will result in mb_free_blocks(inode, e4b, 0, 0)
triggering the BUG_ON on mb_free_blocks():

	BUG_ON(last >= (sb->s_blocksize << 3));

Fix this by bailing out of ext4_discard_allocated_blocks() if fs_len
is zero.

Also fix a missing ext4_mb_unload_buddy() call in
ext4_discard_allocated_blocks().

Google-Bug-Id: 16844242

Fixes: 86f0afd463215fc3e58020493482faa4ac3a4d69
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 818b43e..5baa7ba 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1312,6 +1312,8 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
        void *buddy2;
        struct super_block *sb = e4b->bd_sb;
 
+       if (WARN_ON(count == 0))
+               return;
        BUG_ON(first + count > (sb->s_blocksize << 3));
        assert_spin_locked(ext4_group_lock_ptr(sb, e4b->bd_group));
        mb_check_buddy(e4b);
@@ -3132,6 +3134,8 @@ static void ext4_discard_allocated_blocks(struct ext4_allocation_context *ac)
        int err;
 
        if (pa == NULL) {
+               if (ac->ac_f_ex.fe_len == 0)
+                       return;
                err = ext4_mb_load_buddy(ac->ac_sb, ac->ac_f_ex.fe_group, &e4b);
                if (err) {
                        /*
@@ -3146,6 +3150,7 @@ static void ext4_discard_allocated_blocks(struct ext4_allocation_context *ac)
                mb_free_blocks(ac->ac_inode, &e4b, ac->ac_f_ex.fe_start,
                               ac->ac_f_ex.fe_len);
                ext4_unlock_group(ac->ac_sb, ac->ac_f_ex.fe_group);
+               ext4_mb_unload_buddy(&e4b);
                return;
        }
        if (pa->pa_type == MB_INODE_PA)