From: Tejun Heo tj@kernel.org
commit edaa26334c117a584add6053f48d63a988d25a6e upstream.
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@kernel.org Fixes: f1de2439ec43 ("blk-iocost: revamp donation amount determination") Cc: stable@vger.kernel.org # v5.10+ Link: https://lore.kernel.org/r/Ybfh86iSvpWKxhVM@slm.duckdns.org Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- block/blk-iocost.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
--- a/block/blk-iocost.c +++ b/block/blk-iocost.c @@ -2246,7 +2246,14 @@ static void ioc_timer_fn(struct timer_li 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);