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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 24 Sep 2020 00:06:03 -0700
Message-ID: <CALvZod6v2oYk+xhAzM97-wPRdHmCv-C5zEctXzBk=wd0ZAKruw@mail.gmail.com>
Subject: Re: [PATCH v1 4/4] mm: convert page kmemcg type to a page memcg flag
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, Kernel Team <kernel-team@fb.com>
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On Tue, Sep 22, 2020 at 1:37 PM Roman Gushchin <guro@fb.com> wrote:
>
> PageKmemcg flag is currently defined as a page type (like buddy,
> offline, table and guard). Semantically it means that the page
> was accounted as a kernel memory by the page allocator and has
> to be uncharged on the release.
>
> As a side effect of defining the flag as a page type, the accounted
> page can't be mapped to userspace (look at page_has_type() and
> comments above). In particular, this blocks the accounting of
> vmalloc-backed memory used by some bpf maps, because these maps
> do map the memory to userspace.
>
> One option is to fix it by complicating the access to page->mapcount,
> which provides some free bits for page->page_type.
>
> But it's way better to move this flag into page->memcg_data flags.
> Indeed, the flag makes no sense without enabled memory cgroups
> and memory cgroup pointer set in particular.
>
> This commit replaces PageKmemcg() and __SetPageKmemcg() with
> PageMemcgKmem() and SetPageMemcgKmem(). __ClearPageKmemcg()
> can be simple deleted because clear_page_mem_cgroup() already
> does the job.
>
> As a bonus, on !CONFIG_MEMCG build the PageMemcgKmem() check will
> be compiled out.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>