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.