Patch "btrfs: account for non-CoW'd blocks in btrfs_abort_transaction" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

btrfs: account for non-CoW'd blocks in btrfs_abort_transaction

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-account-for-non-cow-d-blocks-in-btrfs_abort_transaction.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jeff Mahoney <[email protected]>
Date: Wed, 8 Jun 2016 00:36:38 -0400
Subject: btrfs: account for non-CoW'd blocks in btrfs_abort_transaction

From: Jeff Mahoney <[email protected]>

commit 64c12921e11b3a0c10d088606e328c58e29274d8 upstream.

The test for !trans->blocks_used in btrfs_abort_transaction is insufficient to determine whether it's safe to drop the transaction handle on the floor. btrfs_cow_block, informed by should_cow_block, can return blocks that have already been CoW'd in the current transaction. trans->blocks_used is only incremented for new block allocations. If an operation overlaps the blocks in the current transaction entirely and must abort the transaction, we'll happily let it clean up the trans handle even though it may have modified the blocks and will commit an incomplete operation.

In the long-term, I'd like to do closer tracking of when the fs is actually modified so we can still recover as gracefully as possible, but that approach will need some discussion. In the short term, since this is the only code using trans->blocks_used, let's just switch it to a bool indicating whether any blocks were used and set it when should_cow_block returns false.

---
 fs/btrfs/ctree.c       |    5 ++++-
 fs/btrfs/extent-tree.c |    2 +-
 fs/btrfs/super.c       |    2 +-
 fs/btrfs/transaction.h |    2 +-
 4 files changed, 7 insertions(+), 4 deletions(-)

@@ -1551,6 +1551,7 @@ noinline int btrfs_cow_block(struct btrf
 		       trans->transid, root->fs_info->generation);
 
 	if (!should_cow_block(trans, root, buf)) {
+		trans->dirty = true;
 		*cow_ret = buf;
 		return 0;
 	}

@@ -2773,8 +2774,10 @@ again:
 			 * then we don't want to set the path blocking,
 			 * so we test it here
 			 */
-			if (!should_cow_block(trans, root, b))
+			if (!should_cow_block(trans, root, b)) {
+				trans->dirty = true;
 				goto cow_done;
+			}
 
 			/*
 			 * must have write locks on this node and the

@@ -7856,7 +7856,7 @@ btrfs_init_new_buffer(struct btrfs_trans
 		set_extent_dirty(&trans->transaction->dirty_pages, buf->start,
 			 buf->start + buf->len - 1, GFP_NOFS);
 	}
-	trans->blocks_used++;
+	trans->dirty = true;
 	/* this returns a buffer locked for blocking */
 	return buf;
 }

@@ -239,7 +239,7 @@ void __btrfs_abort_transaction(struct bt
 	trans->aborted = errno;
 	/* Nothing used. The other threads that have joined this
 	 * transaction may be able to continue. */
-	if (!trans->blocks_used && list_empty(&trans->new_bgs)) {
+	if (!trans->dirty && list_empty(&trans->new_bgs)) {
 		const char *errstr;
 
 		errstr = btrfs_decode_error(errno);

@@ -110,7 +110,6 @@ struct btrfs_trans_handle {
 	u64 chunk_bytes_reserved;
 	unsigned long use_count;
 	unsigned long blocks_reserved;
-	unsigned long blocks_used;
 	unsigned long delayed_ref_updates;
 	struct btrfs_transaction *transaction;
 	struct btrfs_block_rsv *block_rsv;

@@ -121,6 +120,7 @@ struct btrfs_trans_handle {
 	bool can_flush_pending_bgs;
 	bool reloc_reserved;
 	bool sync;
+	bool dirty;
 	unsigned int type;
 	/*
 	 * this root is only needed to validate that the root passed to

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/btrfs-account-for-non-cow-d-blocks-in-btrfs_abort_transaction.patch