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    SubjectRe: [PATCH 1/3] mm/vmscan: restore zone_reclaim_mode ABI
    On Wed, 1 Jul 2020, Dave Hansen wrote:

    >
    > From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
    > Fixes: 648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE")
    > Cc: Ben Widawsky <ben.widawsky@intel.com>
    > Cc: Alex Shi <alex.shi@linux.alibaba.com>
    > Cc: Daniel Wagner <dwagner@suse.de>
    > Cc: "Tobin C. Harding" <tobin@kernel.org>
    > Cc: Christoph Lameter <cl@linux.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Huang Ying <ying.huang@intel.com>
    > Cc: Dan Williams <dan.j.williams@intel.com>
    > Cc: Qian Cai <cai@lca.pw>
    > Cc: Daniel Wagner <dwagner@suse.de>
    > Cc: stable@vger.kernel.org

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

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