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

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Date:   Fri, 26 Jun 2020 15:24:26 -0400
From:   Qian Cai <cai@....pw>
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, stable@...nel.org
Subject: Re: [PATCH] mm/vmscan: restore zone_reclaim_mode ABI

On Thu, Jun 25, 2020 at 05:34:59PM -0700, 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 evidently stopped caring about RECLAIM_ZONE at some point (or
> never cared) and the #define itself was later removed as a cleanup.
> Those things by themselves are fine.
> 
> But, 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: commit 648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE")

This is a wrong format.