Re: [PATCH v2] btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes
On Wed, Jan 24, 2018 at 11:33:08AM +0800, Lu Fengqi wrote:
> 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 WL 4.11.9-zb64+ #1
> > Hardware name: System manufacturer System Product Name/M5A78L-M/USB3,
> > BIOS 210112/02/2014
> > 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
> > on the linux-btrfs mailing list.
> >
> >Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()")
> >Signed-off-by: Zygo Blaxell
>
> Reviewed-by: Lu Fengqi
Added to next, thanks.
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Re: [PATCH v2] btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes
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 WL 4.11.9-zb64+ #1
> Hardware name: System manufacturer System Product Name/M5A78L-M/USB3,
> BIOS 210112/02/2014
> 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
> on the linux-btrfs mailing list.
>
>Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()")
>Signed-off-by: Zygo Blaxell
Reviewed-by: Lu Fengqi
--
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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[PATCH v2] btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes
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 WL 4.11.9-zb64+ #1
Hardware name: System manufacturer System Product Name/M5A78L-M/USB3,
BIOS 210112/02/2014
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
on the linux-btrfs mailing list.
Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()")
Signed-off-by: Zygo Blaxell
---
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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