ACK: [SRU Groovy] UBUNTU: SAUCE: Revert "mm: memcg/slab: optimize objcg stock draining"

[Tim Gardner]

Acked-by: Tim Gardner <tim.gardner at canonical.com>

Seems pretty cut and dried.

On 3/11/21 7:41 AM, Thadeu Lima de Souza Cascardo wrote:
> BugLink: https://bugs.launchpad.net/bugs/1918668
> 
> This reverts commit 482b2134fda0a74aff66147e777233caede3a0bc, which is commit
> 3de7d4f25a7438f09fef4e71ef111f1805cd8e7c upstream.
> 
> It is supposed to fix f2fe7b09a52b ("mm: memcg/slab: charge individual slab
> objects instead of pages"), which is not present in groovy kernel at all.
> 
> It leads to NULL pointer derefences like:
> [ 10.012503] BUG: kernel NULL pointer dereference, address: 0000000000000518
> [ 10.030761] #PF: supervisor read access in kernel mode
> [ 10.042518] #PF: error_code(0x0000) - not-present page
> [ 10.050165] PGD 0 P4D 0
> [ 10.077050] Oops: 0000 [#1] SMP PTI
> [ 10.081927] CPU: 0 PID: 516 Comm: kexec-load Tainted: G W 5.8.0-45-generic #51
> [ 10.092486] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1 04/01/2014
> [ 10.103510] RIP: 0010:__mod_memcg_state.part.0+0xc/0x90
> 
> This is possibly caused by a refcount going past 0:
> [    5.792689] percpu ref (css_release) <= 0 (-81) after switching to atomic
> [    5.792747] WARNING: CPU: 2 PID: 24 at lib/percpu-refcount.c:161 percpu_ref_switch_to_atomic_rcu+0x147/0x160
> 
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> ---
>   mm/memcontrol.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fdfae074ac91..bf0fc20ae20b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2936,7 +2936,9 @@ void __memcg_kmem_uncharge(struct mem_cgroup *memcg, unsigned int nr_pages)
>   	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
>   		page_counter_uncharge(&memcg->kmem, nr_pages);
>   
> -	refill_stock(memcg, nr_pages);
> +	page_counter_uncharge(&memcg->memory, nr_pages);
> +	if (do_memsw_account())
> +		page_counter_uncharge(&memcg->memsw, nr_pages);
>   }
>   
>   /**
> 

-- 
-----------
Tim Gardner
Canonical, Inc