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Date:   Thu, 1 Oct 2020 09:48:54 -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 v4 4/4] mm: convert page kmemcg type to a page memcg flag
Message-ID: <20201001134854.GF469663@cmpxchg.org>
References: <20201001002710.748166-1-guro@fb.com>
 <20201001002710.748166-5-guro@fb.com>
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On Wed, Sep 30, 2020 at 05:27:10PM -0700, Roman Gushchin 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 an open-coded OR operation setting the memcg
> pointer with the MEMCG_DATA_KMEM bit. __ClearPageKmemcg() can be
> simple deleted, as the whole memcg_data is zeroed at once.
> 
> As a bonus, on !CONFIG_MEMCG build the PageMemcgKmem() check will
> be compiled out.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>

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