Patch "btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes" has been added to the 4.14-stable tree

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

    btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes

to the 4.14-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-remove-spurious-warn_on-ref-count-0-in-find_parent_nodes.patch
and it can be found in the queue-4.14 subdirectory.

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

From: Zygo Blaxell <[email protected]>
Date: Tue, 23 Jan 2018 22:22:09 -0500
Subject: btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes

From: Zygo Blaxell <[email protected]>

commit c8195a7b1ad5648857ce20ba24f384faed8512bc upstream.

Until v4.14, this warning was very infrequent:

	WARNING: CPU: 3 PID: 18172 at fs/btrfs/backref.c:1391 find_parent_nodes+0xc41/0x14e0
	Modules linked in: [...]
	CPU: 3 PID: 18172 Comm: bees Tainted: G      D W    L  4.11.9-zb64+ #1
	Hardware name: System manufacturer System Product Name/M5A78L-M/USB3, BIOS 2101    12/02/2014
	Call Trace:
	 dump_stack+0x85/0xc2
	 __warn+0xd1/0xf0
	 warn_slowpath_null+0x1d/0x20
	 find_parent_nodes+0xc41/0x14e0
	 __btrfs_find_all_roots+0xad/0x120
	 ? extent_same_check_offsets+0x70/0x70
	 iterate_extent_inodes+0x168/0x300
	 iterate_inodes_from_logical+0x87/0xb0
	 ? iterate_inodes_from_logical+0x87/0xb0
	 ? extent_same_check_offsets+0x70/0x70
	 btrfs_ioctl+0x8ac/0x2820
	 ? lock_acquire+0xc2/0x200
	 do_vfs_ioctl+0x91/0x700
	 ? __fget+0x112/0x200
	 SyS_ioctl+0x79/0x90
	 entry_SYSCALL_64_fastpath+0x23/0xc6
	 ? trace_hardirqs_off_caller+0x1f/0x140

Starting with v4.14 (specifically 86d5f9944252 ("btrfs: convert prelimary reference tracking to use rbtrees")) the WARN_ON occurs three orders of magnitude more frequently--almost once per second while running workloads like bees.

Replace the WARN_ON() with a comment rationale for its removal. The rationale is paraphrased from an explanation by Edmund Nadolski <[email protected]> on the linux-btrfs mailing list.

---
 fs/btrfs/backref.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -1252,7 +1252,16 @@ again:
 	while (node) {
 		ref = rb_entry(node, struct prelim_ref, rbnode);
 		node = rb_next(&ref->rbnode);
-		WARN_ON(ref->count < 0);
+		/*
+		 * ref->count < 0 can happen here if there are delayed
+		 * refs with a node->action of BTRFS_DROP_DELAYED_REF.
+		 * prelim_ref_insert() relies on this when merging
+		 * identical refs to keep the overall count correct.
+		 * prelim_ref_insert() will merge only those refs
+		 * which compare identically.  Any refs having
+		 * e.g. different offsets would not be merged,
+		 * and would retain their original ref->count < 0.
+		 */
 		if (roots && ref->count && ref->root_id && ref->parent == 0) {
 			if (sc && sc->root_objectid &&
 			    ref->root_id != sc->root_objectid) {

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

queue-4.14/btrfs-remove-spurious-warn_on-ref-count-0-in-find_parent_nodes.patch