On Thu, Sep 10, 2020 at 1:27 PM Roman Gushchin <guro@xxxxxx> 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 introduces 4 new helper functions and converts all > raw accesses to page->mem_cgroup to calls of these helpers: > struct mem_cgroup *page_mem_cgroup(struct page *page); > struct mem_cgroup *page_mem_cgroup_check(struct page *page); > void set_page_mem_cgroup(struct page *page, struct mem_cgroup *memcg); > void clear_page_mem_cgroup(struct page *page); > > page_mem_cgroup_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_mem_cgroup() contains a VM_BUG_ON_PAGE() check for the page not > being a slab page. So do set_page_mem_cgroup() and clear_page_mem_cgroup(). > > 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. > > page_memcg() and page_memcg_rcu() helpers defined in mm.h are removed. > New page_mem_cgroup() is a direct analog of page_memcg(), while > page_memcg_rcu() has a single call site in a small rcu-read-lock > section, so it's just not worth it to have a separate helper. So > it's replaced with page_mem_cgroup() too. > > Signed-off-by: Roman Gushchin <guro@xxxxxx> You need to update a couple of comments in kernel/fork.c, mm/slab.h, mm/workingset.c, fs/buffer.c, fs/iomap/buffered-io.c. Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
