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Subject: Re: [PATCH v3 02/19] mm: memcg: prepare for byte-sized vmstat items
From:   Vlastimil Babka <vbabka@suse.cz>
To:     Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
References: <20200422204708.2176080-1-guro@fb.com>
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Message-ID: <ab1e0eae-9baf-eccf-82cb-9e47dbbab13b@suse.cz>
Date:   Wed, 20 May 2020 13:36:09 +0200
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On 5/20/20 1:31 PM, Vlastimil Babka wrote:
> On 4/22/20 10:46 PM, Roman Gushchin wrote:
>> To implement per-object slab memory accounting, we need to
>> convert slab vmstat counters to bytes. Actually, out of
>> 4 levels of counters: global, per-node, per-memcg and per-lruvec
>> only two last levels will require byte-sized counters.
>> It's because global and per-node counters will be counting the
>> number of slab pages, and per-memcg and per-lruvec will be
>> counting the amount of memory taken by charged slab objects.
>> 
>> Converting all vmstat counters to bytes or even all slab
>> counters to bytes would introduce an additional overhead.
>> So instead let's store global and per-node counters
>> in pages, and memcg and lruvec counters in bytes.
>> 
>> To make the API clean all access helpers (both on the read
>> and write sides) are dealing with bytes.
>> 
>> To avoid back-and-forth conversions a new flavor of helpers
>> is introduced, which always returns values in pages:
>> node_page_state_pages() and global_node_page_state_pages().
>> 
>> Actually new helpers are just reading raw values. Old helpers are
>> simple wrappers, which perform a conversion if the vmstat items are
>> in bytes. Because at the moment no one actually need bytes,
>> there are WARN_ON_ONCE() macroses inside to warn about inappropriate
>> use cases.
>> 
>> Thanks to Johannes Weiner for the idea of having the byte-sized API
>> on top of the page-sized internal storage.
>> 
>> Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Reviewed-By: Vlastimil Babka <vbabka@suse.cz>
> 
> But it's somewhat complicated, so it would be great to document it in comments
> of e.g. include/linux/vmstat.h that what the API returns as unsigned long, can
> be either bytes or pages depending on vmstat_item_in_bytes().

Also forgot to add that if those WARN_ON_ONCEs are going to stay, they should
rather become VM_WARN_ON_ONCEs