Commit ad7fa852 authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
Browse files

memcg: implement mem_cgroup_css_from_page()

Implement mem_cgroup_css_from_page() which returns the
cgroup_subsys_state of the memcg associated with a given page on the
default hierarchy.  This will be used by cgroup writeback support.

This function assumes that page->mem_cgroup association doesn't change
until the page is released, which is true on the default hierarchy as
long as replace_page_cache_page() is not used.  As the only user of
replace_page_cache_page() is FUSE which won't support cgroup writeback
for the time being, this works for now, and replace_page_cache_page()
will soon be updated so that the invariant actually holds.

Note that the RCU protected page->mem_cgroup access is consistent with
other usages across memcg but ultimately incorrect.  These unlocked
accesses are missing required barriers.  page->mem_cgroup should be
made an RCU pointer and updated and accessed using RCU operations.

v4: Instead of triggering WARN, return the root css on the traditional
    hierarchies.  This makes the function a lot easier to deal with
    especially as there's no light way to synchronize against
    hierarchy rebinding.

v3: s/mem_cgroup_migrate()/mem_cgroup_css_from_page()/

v2: Trigger WARN if the function is used on the traditional
    hierarchies and add comment about the assumed invariant.
Signed-off-by: default avatarTejun Heo <>
Cc: Johannes Weiner <>
Cc: Michal Hocko <>
Signed-off-by: default avatarJens Axboe <>
parent 1d933cf0
......@@ -115,6 +115,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
extern struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg);
extern struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page);
struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
struct mem_cgroup *,
......@@ -598,6 +598,39 @@ struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg)
return &memcg->css;
* mem_cgroup_css_from_page - css of the memcg associated with a page
* @page: page of interest
* If memcg is bound to the default hierarchy, css of the memcg associated
* with @page is returned. The returned css remains associated with @page
* until it is released.
* If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup
* is returned.
* XXX: The above description of behavior on the default hierarchy isn't
* strictly true yet as replace_page_cache_page() can modify the
* association before @page is released even on the default hierarchy;
* however, the current and planned usages don't mix the the two functions
* and replace_page_cache_page() will soon be updated to make the invariant
* actually true.
struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
struct mem_cgroup *memcg;
memcg = page->mem_cgroup;
if (!memcg || !cgroup_on_dfl(memcg->css.cgroup))
memcg = root_mem_cgroup;
return &memcg->css;
static struct mem_cgroup_per_zone *
mem_cgroup_page_zoneinfo(struct mem_cgroup *memcg, struct page *page)
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