Re: [PATCH] mm, memcg: Do not high throttle allocators based on wraparound

From: Chris Down
Date: Tue Mar 31 2020 - 15:00:25 EST


Chris Down writes:
Michal Hocko writes:
I find this paragraph rather confusing. This is essentially an unsigned
underflow when any of the memcg up the hierarchy is below the high
limit, right? There doesn't really seem anything complex in such a
hierarchy.

The conditions to trigger the bug itself are easy, but having it obviously visible in tests requires a moderately complex hierarchy, since in the basic case ancestor_usage is "similar enough" to the test leaf cgroup's usage.

Here is another reason why this wasn't caught -- division usually renders the overage 0 anyway with such a large input.

With the attached patch applied before this fix, you can see that usually division results in an overage of 0, so the result is the same. Here's an example where pid 213 is a cgroup in system.slice/foo.service hitting its own memory.high, and system.slice has no memory.high configuresd:

[root@ktst ~]# cat /sys/kernel/debug/tracing/trace
# tracer: nop
#
# entries-in-buffer/entries-written: 33/33 #P:4
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
(bash)-213 [002] .N.. 58.873988: mem_cgroup_handle_over_high: usage: 32, high: 1
(bash)-213 [002] .N.. 58.873993: mem_cgroup_handle_over_high: 1 overage before shifting (31)
(bash)-213 [002] .N.. 58.873994: mem_cgroup_handle_over_high: 1 overage after shifting (32505856)
(bash)-213 [002] .N.. 58.873995: mem_cgroup_handle_over_high: 1 overage after div (32505856)
(bash)-213 [002] .N.. 58.873996: mem_cgroup_handle_over_high: 1 cgroup new overage (32505856)
(bash)-213 [002] .N.. 58.873998: mem_cgroup_handle_over_high: usage: 18641, high: 2251799813685247
(bash)-213 [002] .N.. 58.873998: mem_cgroup_handle_over_high: 2 overage before shifting (18444492273895885010)
(bash)-213 [002] .N.. 58.873999: mem_cgroup_handle_over_high: 2 overage after shifting (19547553792)
(bash)-213 [002] .N.. 58.874000: mem_cgroup_handle_over_high: 2 overage after div (0)
(bash)-213 [002] .N.. 58.874001: mem_cgroup_handle_over_high: 2 cgroup too low (0)
(bash)-213 [002] .N.. 58.874002: mem_cgroup_handle_over_high: Used 1 from leaf to get result