From: Roman Gushchin guro@fb.com Subject: mm: slab: make page_cgroup_ino() to recognize non-compound slab pages properly

page_cgroup_ino() doesn't return a valid memcg pointer for non-compound slab pages, because it depends on PgHead AND PgSlab flags to be set to determine the memory cgroup from the kmem_cache. It's correct for compound pages, but not for generic small pages. Those don't have PgHead set, so it ends up returning zero.

Fix this by replacing the condition to PageSlab() && !PageTail().

Before this patch: [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@0.service/ | grep slab 0x0000000000000080 38 0 _______S___________________________________ slab

After this patch: [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@0.service/ | grep slab 0x0000000000000080 147 0 _______S___________________________________ slab

Also, hwpoison_filter_task() uses output of page_cgroup_ino() in order to filter error injection events based on memcg. So if page_cgroup_ino() fails to return memcg pointer, we just fail to inject memory error. Considering that hwpoison filter is for testing, affected users are limited and the impact should be marginal.

[n-horiguchi@ah.jp.nec.com: changelog additions] Link: http://lkml.kernel.org/r/20191031012151.2722280-1-guro@fb.com Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages") Signed-off-by: Roman Gushchin guro@fb.com Reviewed-by: Shakeel Butt shakeelb@google.com Acked-by: David Rientjes rientjes@google.com Cc: Vladimir Davydov vdavydov.dev@gmail.com Cc: Daniel Jordan daniel.m.jordan@oracle.com Cc: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 2 +- mm/slab.h | 4 ++-- 2 files changed, 3 insertions(+), 3 deletions(-)

--- a/mm/memcontrol.c~mm-slab-make-page_cgroup_ino-to-recognize-non-compound-slab-pages-properly +++ a/mm/memcontrol.c @@ -484,7 +484,7 @@ ino_t page_cgroup_ino(struct page *page) unsigned long ino = 0;

rcu_read_lock(); - if (PageHead(page) && PageSlab(page)) + if (PageSlab(page) && !PageTail(page)) memcg = memcg_from_slab_page(page); else memcg = READ_ONCE(page->mem_cgroup); --- a/mm/slab.h~mm-slab-make-page_cgroup_ino-to-recognize-non-compound-slab-pages-properly +++ a/mm/slab.h @@ -323,8 +323,8 @@ static inline struct kmem_cache *memcg_r * Expects a pointer to a slab page. Please note, that PageSlab() check * isn't sufficient, as it returns true also for tail compound slab pages, * which do not have slab_cache pointer set. - * So this function assumes that the page can pass PageHead() and PageSlab() - * checks. + * So this function assumes that the page can pass PageSlab() && !PageTail() + * check. * * The kmem_cache can be reparented asynchronously. The caller must ensure * the memcg lifetime, e.g. by taking rcu_read_lock() or cgroup_mutex. _