On Tue, Jan 23, 2018 at 10:22:09PM -0500, Zygo Blaxell wrote:
>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.
>
>Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()")
>Signed-off-by: Zygo Blaxell <[email protected]>
Reviewed-by: Lu Fengqi <[email protected]>
--
Thanks,
Lu
>---
>v2:
> Replace WARN_ON with rationale instead of merely deleting it.
> Trim irrelevant detail from the backtrace. Add Fixes reference.
> Fix subject line (missing "< 0").
>
> fs/btrfs/backref.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
>diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
>index 7d0dc100a09a..06597c5f9f4b 100644
>--- a/fs/btrfs/backref.c
>+++ b/fs/btrfs/backref.c
>@@ -1263,7 +1263,16 @@ static int find_parent_nodes(struct btrfs_trans_handle
>*trans,
> 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) {
>--
>2.11.0
>
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