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

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Message-ID: <0c1e01bb-bd10-50ab-ded0-9d4f37951f25@intel.com>
Date:   Thu, 2 Jul 2020 07:36:56 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        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, dan.j.williams@...el.com, cai@....pw,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/vmscan: restore zone_reclaim_mode ABI

On 7/2/20 4:28 AM, Huang, Ying wrote:
>> 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
> Per my understanding, this script would have enabled node reclaim for
> clean unmapped pages before commit 648b5cf368e0 ("mm/vmscan: remove
> unused RECLAIM_OFF/RECLAIM_ZONE").  So we should revise the description
> here?

Yes, you're right.  I updated the patch with the updated understanding
about the implicit use of the bit but didn't update the changelog.  I'll
do that for v3.