linux-kernel - Re: [PATCH v2 08/28] mm: fork: fix kernel_stack memcg stats for various stack implementations

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Date:   Mon, 3 Feb 2020 11:12:00 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 08/28] mm: fork: fix kernel_stack memcg stats for
 various stack implementations

On Mon, Jan 27, 2020 at 09:34:33AM -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.
> 
> 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 .

The change looks good to me, but it sounds like this is a bug with
actual consequences to userspace. Can you elaborate on that in the
changelog please? Maybe add a Fixes: line, if applicable?