linux-kernel - Re: [RESEND PATCH 1/2 -mm] mm: account lazy free pages separately

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Message-ID: <564a0860-94f1-6301-5527-5c2272931d8b@suse.cz>
Date:   Wed, 14 Aug 2019 14:49:18 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     kirill.shutemov@...ux.intel.com, hannes@...xchg.org,
        rientjes@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Linux API <linux-api@...r.kernel.org>
Subject: Re: [RESEND PATCH 1/2 -mm] mm: account lazy free pages separately

On 8/9/19 8:26 PM, Yang Shi wrote:
> Here the new counter is introduced for patch 2/2 to account deferred 
> split THPs into available memory since NR_ANON_THPS may contain 
> non-deferred split THPs.
> 
> I could use an internal counter for deferred split THPs, but if it is 
> accounted by mod_node_page_state, why not just show it in /proc/meminfo? 

The answer to "Why not" is that it becomes part of userspace API (btw this
patchset should have CC'd linux-api@ - please do for further iterations) and
even if the implementation detail of deferred splitting might change in the
future, we'll basically have to keep the counter (even with 0 value) in
/proc/meminfo forever.

Also, quite recently we have added the following counter:

KReclaimable: Kernel allocations that the kernel will attempt to reclaim
              under memory pressure. Includes SReclaimable (below), and other
              direct allocations with a shrinker.

Although THP allocations are not exactly "kernel allocations", once they are
unmapped, they are in fact kernel-only, so IMHO it wouldn't be a big stretch to
add the lazy THP pages there?

> Or we fix NR_ANON_THPS and show deferred split THPs in /proc/meminfo?
> 
>>
>