ext4: Issue the discard operation *before* releasing the blocks to be reused

Otherwise, we can end up having data corruption because the blocks
could get reused and then discarded!

https://bugzilla.kernel.org/show_bug.cgi?id=15579

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 11 insertions, 10 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 54df209d2eed..e5ab41b559c0 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2534,6 +2534,17 @@ static void release_blocks_on_commit(journal_t *journal, transaction_t *txn)
                 mb_debug(1, "gonna free %u blocks in group %u (0x%p):",
                          entry->count, entry->group, entry);
 
+                if (test_opt(sb, DISCARD)) {
+                        ext4_fsblk_t discard_block;
+
+                        discard_block = entry->start_blk +
+                                ext4_group_first_block_no(sb, entry->group);
+                        trace_ext4_discard_blocks(sb,
+                                        (unsigned long long)discard_block,
+                                        entry->count);
+                        sb_issue_discard(sb, discard_block, entry->count);
+                }
+
                 err = ext4_mb_load_buddy(sb, entry->group, &e4b);
                 /* we expect to find existing buddy because it's pinned */
                 BUG_ON(err != 0);
@@ -2555,16 +2566,6 @@ static void release_blocks_on_commit(journal_t *journal, transaction_t *txn)
                         page_cache_release(e4b.bd_bitmap_page);
                 }
                 ext4_unlock_group(sb, entry->group);
-                if (test_opt(sb, DISCARD)) {
-                        ext4_fsblk_t discard_block;
-
-                        discard_block = entry->start_blk +
-                                ext4_group_first_block_no(sb, entry->group);
-                        trace_ext4_discard_blocks(sb,
-                                        (unsigned long long)discard_block,
-                                        entry->count);
-                        sb_issue_discard(sb, discard_block, entry->count);
-                }
                 kmem_cache_free(ext4_free_ext_cachep, entry);
                 ext4_mb_release_desc(&e4b);
         }