On Tue, Sep 29, 2020 at 04:59:17PM -0700, Roman Gushchin wrote: > Currently there are many open-coded reads and writes of the > page->mem_cgroup pointer, as well as a couple of read helpers, > which are barely used. > > It creates an obstacle on a way to reuse some bits of the pointer > for storing additional bits of information. In fact, we already do > this for slab pages, where the last bit indicates that a pointer has > an attached vector of objcg pointers instead of a regular memcg > pointer. > > This commits uses 2 existing helpers and introduces 3 new helpers to > converts all raw accesses to page->mem_cgroup to calls of these helpers: > struct mem_cgroup *page_memcg(struct page *page); > struct mem_cgroup *page_memcg_rcu(struct page *page); > struct mem_cgroup *page_memcg_check(struct page *page); > void set_page_memcg(struct page *page, struct mem_cgroup *memcg); > void clear_page_memcg(struct page *page); > > page_memcg_check() is intended to be used in cases when the page > can be a slab page and have a memcg pointer pointing at objcg vector. > It does check the lowest bit, and if set, returns NULL. > page_memcg() contains a VM_BUG_ON_PAGE() check for the page not > being a slab page. So do set_page_memcg() and clear_page_memcg(). > > To make sure nobody uses a direct access, struct page's > mem_cgroup/obj_cgroups is converted to unsigned long memcg_data. > Only new helpers and a couple of slab-accounting related functions > access this field directly. > > Signed-off-by: Roman Gushchin <guro@xxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
