Re: [PATCH] mm: page_io: fix psi memory pressure error on cold swapins

[Andrew Morton <akpm@linux-foundation.org>]

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?