The donation calculation logic assumes that the donor has non-zero
after-donation hweight, so the lowest active hweight a donating cgroup can
have is 2 so that it can donate 1 while keeping the other 1 for itself.
Earlier, we only donated from cgroups with sizable surpluses so this
condition was always true. However, with the precise donation algorithm
implemented, f1de2439ec43 ("blk-iocost: revamp donation amount
determination") made the donation amount calculation exact enabling even low
hweight cgroups to donate.
This means that in rare occasions, a cgroup with active hweight of 1 can
enter donation calculation triggering the following warning and then a
divide-by-zero oops.
WARNING: CPU: 4 PID: 0 at block/blk-iocost.c:1928 transfer_surpluses.cold+0x0/0x53 [884/94867]
...
RIP: 0010:transfer_surpluses.cold+0x0/0x53
Code: 92 ff 48 c7 c7 28 d1 ab b5 65 48 8b 34 25 00 ae 01 00 48 81 c6 90 06 00 00 e8 8b 3f fe ff 48 c7 c0 ea ff ff ff e9 95 ff 92 ff <0f> 0b 48 c7 c7 30 da ab b5 e8 71 3f fe ff 4c 89 e8 4d 85 ed 74 0
4
...
Call Trace:
<IRQ>
ioc_timer_fn+0x1043/0x1390
call_timer_fn+0xa1/0x2c0
__run_timers.part.0+0x1ec/0x2e0
run_timer_softirq+0x35/0x70
...
iocg: invalid donation weights in /a/b: active=1 donating=1 after=0
Fix it by excluding cgroups w/ active hweight < 2 from donating. Excluding
these extreme low hweight donations shouldn't affect work conservation in
any meaningful way.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Fixes: f1de2439ec43 ("blk-iocost: revamp donation amount determination")
Cc: stable@xxxxxxxxxxxxxxx # v5.10+
---
block/blk-iocost.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index a5b37cc65b171..769b643942989 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -2311,7 +2311,14 @@ static void ioc_timer_fn(struct timer_list *timer)
hwm = current_hweight_max(iocg);
new_hwi = hweight_after_donation(iocg, old_hwi, hwm,
usage, &now);
- if (new_hwi < hwm) {
+ /*
+ * Donation calculation assumes hweight_after_donation
+ * to be positive, a condition that a donor w/ hwa < 2
+ * can't meet. Don't bother with donation if hwa is
+ * below 2. It's not gonna make a meaningful difference
+ * anyway.
+ */
+ if (new_hwi < hwm && hwa >= 2) {
iocg->hweight_donating = hwa;
iocg->hweight_after_donation = new_hwi;
list_add(&iocg->surplus_list, &surpluses);
