Re: [PATCH rfc 2/5] mm: memcontrol: use helpers to access page's memcg data

[Shakeel Butt <shakeelb@google.com>]

On Thu, Sep 10, 2020 at 1:27 PM Roman Gushchin <guro@fb.com> 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@fb.com>

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@google.com>