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Date:   Mon, 14 Feb 2022 14:48:05 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, CGEL <cgel.zte@gmail.com>,
        Minchan Kim <minchan@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Yu Zhao <yuzhao@google.com>
Subject: Re: [PATCH] mm: page_io: fix psi memory pressure error on cold
 swapins
Message-Id: <20220214144805.fa389f495fdfb07b40526f70@linux-foundation.org>
In-Reply-To: <20220214214921.419687-1-hannes@cmpxchg.org>
References: <20220214214921.419687-1-hannes@cmpxchg.org>
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On Mon, 14 Feb 2022 16:49:21 -0500 Johannes Weiner <hannes@cmpxchg.org> wrote:

> Once upon a time, all swapins counted toward memory pressure[1]. Then
> Joonsoo introduced workingset detection for anonymous pages and we
> gained the ability to distinguish hot from cold swapins[2][3]. But we
> failed to update swap_readpage() accordingly, and now we account
> partial memory pressure in the swapin path of cold memory.
> 
> Not for all situations - which adds more inconsistency: paths using
> the conventional submit_bio() and lock_page() route will not see much
> pressure - unless storage itself is heavily congested and the bio
> submissions stall. ZRAM and ZSWAP do most of the work directly from
> swap_readpage() and will see all swapins reflected as pressure.
> 
> Restore consistency by making all swapin stall accounting conditional
> on the page actually being part of the workingset.

Does this have any known runtime effects?  If not, can we
hazard a guess?