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

[Shakeel Butt <shakeelb@google.com>]

+Suleiman Souhlal

On Tue, Oct 22, 2019 at 4:37 PM Johannes Weiner <hannes@cmpxchg.org> 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.
>
> 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.
>
> Cc: stable@kernel.org # 4.18+
> Fixes: e699e2c6a654 ("net, mm: account sock objects to kmemcg")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
>  mm/memcontrol.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8090b4c99ac7..c7e3e758c165 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2528,6 +2528,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>                 goto retry;
>         }
>
> +       /*
> +        * Memcg doesn't have a dedicated reserve for atomic
> +        * allocations. But like the global atomic pool, we need to
> +        * put the burden of reclaim on regular allocation requests
> +        * and let these go through as privileged allocations.
> +        */
> +       if (gfp_mask & __GFP_ATOMIC)
> +               goto force;
> +

Actually we (Google) already have a similar internal patch where we
check for __GFP_HIGH and then go for force charging with similar
reasoning.

>         /*
>          * Unlike in global OOM situations, memcg is not in a physical
>          * memory shortage.  Allow dying and OOM-killed tasks to
> --
> 2.23.0
>