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Date:   Wed, 30 Sep 2020 16:32:23 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, kernel-team@fb.com
Subject: Re: [PATCH v3 1/4] mm: memcontrol: use helpers to access page's
 memcg data
Message-ID: <20200930203223.GA469663@cmpxchg.org>
References: <20200929235920.537849-1-guro@fb.com>
 <20200929235920.537849-2-guro@fb.com>
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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@fb.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>