linux-kernel - Re: [PATCH 1/3] mm/vmscan: restore zone_reclaim

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Message-ID: <alpine.DEB.2.23.453.2007011303080.1908531@chino.kir.corp.google.com>
Date:   Wed, 1 Jul 2020 13:03:20 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        ben.widawsky@...el.com, alex.shi@...ux.alibaba.com,
        dwagner@...e.de, tobin@...nel.org, cl@...ux.com,
        akpm@...ux-foundation.org, ying.huang@...el.com,
        dan.j.williams@...el.com, cai@....pw, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/vmscan: restore zone_reclaim_mode ABI

On Wed, 1 Jul 2020, Dave Hansen wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> I went to go add a new RECLAIM_* mode for the zone_reclaim_mode
> sysctl.  Like a good kernel developer, I also went to go update the
> documentation.  I noticed that the bits in the documentation didn't
> match the bits in the #defines.
> 
> The VM never explicitly checks the RECLAIM_ZONE bit.  The bit is,
> however implicitly checked when checking 'node_reclaim_mode==0'.
> The RECLAIM_ZONE #define was removed in a cleanup.  That, by itself
> is fine.
> 
> But, when the bit was removed (bit 0) the _other_ bit locations also
> got changed.  That's not OK because the bit values are documented to
> mean one specific thing and users surely rely on them meaning that one
> thing and not changing from kernel to kernel.  The end result is that
> if someone had a script that did:
> 
> 	sysctl vm.zone_reclaim_mode=1
> 
> That script went from doing nothing to writing out pages during
> node reclaim after the commit in question.  That's not great.
> 
> Put the bits back the way they were and add a comment so something
> like this is a bit harder to do again.  Update the documentation to
> make it clear that the first bit is ignored.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Fixes: 648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE")
> Cc: Ben Widawsky <ben.widawsky@...el.com>
> Cc: Alex Shi <alex.shi@...ux.alibaba.com>
> Cc: Daniel Wagner <dwagner@...e.de>
> Cc: "Tobin C. Harding" <tobin@...nel.org>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Qian Cai <cai@....pw>
> Cc: Daniel Wagner <dwagner@...e.de>
> Cc: stable@...r.kernel.org

Acked-by: David Rientjes <rientjes@...gle.com>