linux-kernel - Re: [PATCH RFC 04/14] mm: vmstat: convert slab vmstat counter to bytes

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Date:   Mon, 16 Sep 2019 14:38:40 +0200
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH RFC 04/14] mm: vmstat: convert slab vmstat counter to
 bytes

On Thu, Sep 05, 2019 at 02:45:48PM -0700, Roman Gushchin wrote:
> In order to prepare for per-object slab memory accounting,
> convert NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE vmstat
> items to bytes.
> 
> To make sure that these vmstats are in bytes, rename them
> to NR_SLAB_RECLAIMABLE_B and NR_SLAB_UNRECLAIMABLE_B (similar to
> NR_KERNEL_STACK_KB).
> 
> The size of slab memory shouldn't exceed 4Gb on 32-bit machines,
> so it will fit into atomic_long_t we use for vmstats.
> 
> Signed-off-by: Roman Gushchin <guro@...com>

Maybe a crazy idea, but instead of mixing bytes and pages, would it be
difficult to account all vmstat items in bytes internally? And provide
two general apis, byte and page based, to update and query the counts,
instead of tying the unit it to individual items?

The vmstat_item_in_bytes() conditional shifting is pretty awkward in
code that has a recent history littered with subtle breakages.

The translation helper node_page_state_pages() will yield garbage if
used with the page-based counters, which is another easy to misuse
interface.

We already have many places that multiply with PAGE_SIZE to get the
stats in bytes or kb units.

And _B/_KB suffixes are kinda clunky.

The stats use atomic_long_t, so switching to atomic64_t doesn't make a
difference on 64-bit and is backward compatible with 32-bit.

The per-cpu batch size you have to raise from s8 either way.

It seems to me that would make the code and API a lot simpler and
easier to use / harder to misuse.