Re: [PATCH] mm: memcontrol: fix network errors from failing __GFP_ATOMIC charges

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Oct 23, 2019 at 08:40:12AM +0200, Michal Hocko wrote:
> On Tue 22-10-19 19:37:08, Johannes Weiner wrote:
> > While upgrading from 4.16 to 5.2, we noticed these allocation errors
> > in the log of the new kernel:
> > 
> > [ 8642.253395] SLUB: Unable to allocate memory on node -1, gfp=0xa20(GFP_ATOMIC)
> > [ 8642.269170]   cache: tw_sock_TCPv6(960:helper-logs), object size: 232, buffer size: 240, default order: 1, min order: 0
> > [ 8642.293009]   node 0: slabs: 5, objs: 170, free: 0
> > 
> >         slab_out_of_memory+1
> >         ___slab_alloc+969
> >         __slab_alloc+14
> >         kmem_cache_alloc+346
> >         inet_twsk_alloc+60
> >         tcp_time_wait+46
> >         tcp_fin+206
> >         tcp_data_queue+2034
> >         tcp_rcv_state_process+784
> >         tcp_v6_do_rcv+405
> >         __release_sock+118
> >         tcp_close+385
> >         inet_release+46
> >         __sock_release+55
> >         sock_close+17
> >         __fput+170
> >         task_work_run+127
> >         exit_to_usermode_loop+191
> >         do_syscall_64+212
> >         entry_SYSCALL_64_after_hwframe+68
> > 
> > accompanied by an increase in machines going completely radio silent
> > under memory pressure.
> 
> This is really worrying because that suggests that something depends on
> GFP_ATOMIC allocation which is fragile and broken. 

I don't think that is true. You cannot rely on a *single instance* of
atomic allocations to succeed. But you have to be able to rely on that
failure is temporary and there is a chance of succeeding eventually.

Network is a good example. It retries transmits, but within reason. If
you aren't able to process incoming packets for minutes, you might as
well be dead.

> > One thing that changed since 4.16 is e699e2c6a654 ("net, mm: account
> > sock objects to kmemcg"), which made these slab caches subject to
> > cgroup memory accounting and control.
> > 
> > The problem with that is that cgroups, unlike the page allocator, do
> > not maintain dedicated atomic reserves. As a cgroup's usage hovers at
> > its limit, atomic allocations - such as done during network rx - can
> > fail consistently for extended periods of time. The kernel is not able
> > to operate under these conditions.
> > 
> > We don't want to revert the culprit patch, because it indeed tracks a
> > potentially substantial amount of memory used by a cgroup.
> > 
> > We also don't want to implement dedicated atomic reserves for cgroups.
> > There is no point in keeping a fixed margin of unused bytes in the
> > cgroup's memory budget to accomodate a consumer that is impossible to
> > predict - we'd be wasting memory and get into configuration headaches,
> > not unlike what we have going with min_free_kbytes. We do this for
> > physical mem because we have to, but cgroups are an accounting game.
> > 
> > Instead, account these privileged allocations to the cgroup, but let
> > them bypass the configured limit if they have to. This way, we get the
> > benefits of accounting the consumed memory and have it exert pressure
> > on the rest of the cgroup, but like with the page allocator, we shift
> > the burden of reclaimining on behalf of atomic allocations onto the
> > regular allocations that can block.
> 
> On the other hand this would allow to break the isolation by an
> unpredictable amount. Should we put a simple cap on how much we can go
> over the limit. If the memcg limit reclaim is not able to keep up with
> those overflows then even __GFP_ATOMIC allocations have to fail. What do
> you think?

I don't expect a big overrun in practice, and it appears that Google
has been letting even NOWAIT allocations pass through without
isolation issues. Likewise, we have been force-charging the skmem for
a while now and it hasn't been an issue for reclaim to keep up.

My experience from production is that it's a whole lot easier to debug
something like a memory.max overrun than it is to debug a machine that
won't respond to networking. So that's the side I would err on.