Re: [PATCH] net/skbuff: silence warnings under memory pressure

[Michal Hocko]

On Tue 03-09-19 11:42:22, Qian Cai wrote:
> On Tue, 2019-09-03 at 15:22 +0200, Michal Hocko wrote:
> > On Fri 30-08-19 18:15:22, Eric Dumazet wrote:
> > > If there is a risk of flooding the syslog, we should fix this generically
> > > in mm layer, not adding hundred of __GFP_NOWARN all over the places.
> > 
> > We do already ratelimit in warn_alloc. If it isn't sufficient then we
> > can think of a different parameters. Or maybe it is the ratelimiting
> > which doesn't work here. Hard to tell and something to explore.
> 
> The time-based ratelimit won't work for skb_build() as when a system under
> memory pressure, and the CPU is fast and IO is so slow, it could take a long
> time to swap and trigger OOM.

I really do not understand what does OOM and swapping have to do with
the ratelimiting here. The sole purpose of the ratelimit is to reduce
the amount of warnings to be printed. Slow IO might have an effect on
when the OOM killer is invoked but atomic allocations are not directly
dependent on IO.

> I suppose what happens is those skb_build() allocations are from softirq, and
> once one of them failed, it calls printk() which generates more interrupts.
> Hence, the infinite loop.

Please elaborate more.

-- 
Michal Hocko
SUSE Labs