Re: [PATCH] mm: fork: fix kernel_stack memcg stats for various stack implementations

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 03, 2020 at 03:35:50PM -0800, Roman Gushchin wrote:
> Depending on CONFIG_VMAP_STACK and the THREAD_SIZE / PAGE_SIZE ratio
> the space for task stacks can be allocated using __vmalloc_node_range(),
> alloc_pages_node() and kmem_cache_alloc_node(). In the first and the
> second cases page->mem_cgroup pointer is set, but in the third it's
> not: memcg membership of a slab page should be determined using the
> memcg_from_slab_page() function, which looks at
> page->slab_cache->memcg_params.memcg . In this case, using
> mod_memcg_page_state() (as in account_kernel_stack()) is incorrect:
> page->mem_cgroup pointer is NULL even for pages charged to a non-root
> memory cgroup.
> 
> It can lead to kernel_stack per-memcg counters permanently showing 0
> on some architectures (depending on the configuration).
> 
> In order to fix it, let's introduce a mod_memcg_obj_state() helper,
> which takes a pointer to a kernel object as a first argument, uses
> mem_cgroup_from_obj() to get a RCU-protected memcg pointer and
> calls mod_memcg_state(). It allows to handle all possible
> configurations (CONFIG_VMAP_STACK and various THREAD_SIZE/PAGE_SIZE
> values) without spilling any memcg/kmem specifics into fork.c .
> 
> Note: this patch has been first posted as a part of the new slab
> controller patchset. This is a slightly updated version: the fixes
> tag has been added and the commit log was extended by the advice
> of Johannes Weiner. Because it's a fix that makes sense by itself,
> I'm re-posting it as a standalone patch.
> 
> Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: stable@vger.kernel.org

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks for pointing out the user impact.