Re: INFO: rcu detected stall in shmem_fault

[Dmitry Vyukov]

On Wed, Oct 10, 2018 at 2:29 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Wed, Oct 10, 2018 at 2:25 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> On Wed 10-10-18 20:48:33, Sergey Senozhatsky wrote:
>>> On (10/10/18 13:35), Michal Hocko wrote:
>>> > > Just flooding out of memory messages can trigger RCU stall problems.
>>> > > For example, a severe skbuff_head_cache or kmalloc-512 leak bug is causing
>>> >
>>> > [...]
>>> >
>>> > Quite some of them, indeed! I guess we want to rate limit the output.
>>> > What about the following?
>>>
>>> A bit unrelated, but while we are at it:
>>>
>>>   I like it when we rate-limit printk-s that lookup the system.
>>> But it seems that default rate-limit values are not always good enough,
>>> DEFAULT_RATELIMIT_INTERVAL / DEFAULT_RATELIMIT_BURST can still be too
>>> verbose. For instance, when we have a very slow IPMI emulated serial
>>> console -- e.g. baud rate at 57600. DEFAULT_RATELIMIT_INTERVAL and
>>> DEFAULT_RATELIMIT_BURST can add new OOM headers and backtraces faster
>>> than we evict them.
>>>
>>> Does it sound reasonable enough to use larger than default rate-limits
>>> for printk-s in OOM print-outs? OOM reports tend to be somewhat large
>>> and the reported numbers are not always *very* unique.
>>>
>>> What do you think?
>>
>> I do not really care about the current inerval/burst values. This change
>> should be done seprately and ideally with some numbers.
>
> I think Sergey meant that this place may need to use
> larger-than-default values because it prints lots of output per
> instance (whereas the default limit is more tuned for cases that print
> just 1 line).
>
> I've found at least 1 place that uses DEFAULT_RATELIMIT_INTERVAL*10:
> https://elixir.bootlin.com/linux/latest/source/fs/btrfs/extent-tree.c#L8365
> Probably we need something similar here.


In parallel with the kernel changes I've also made a change to
syzkaller that (1) makes it not use oom_score_adj=-1000, this hard
killing limit looks like quite risky thing, (2) increase memcg size
beyond expected KASAN quarantine size:
https://github.com/google/syzkaller/commit/adedaf77a18f3d03d695723c86fc083c3551ff5b
If this will stop the flow of hang/stall reports, then we can just
close all old reports as invalid.