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

On Tue 03-09-19 11:42:22, Qian Cai wrote:

On Tue, 2019-09-03 at 15:22 +0200, Michal Hocko wrote:

quoted

On Fri 30-08-19 18:15:22, Eric Dumazet wrote:

quoted

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