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Wed, 01 Jul 2020 13:03:21 -0700 (PDT) Date: Wed, 1 Jul 2020 13:03:20 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Dave Hansen cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, ben.widawsky@intel.com, alex.shi@linux.alibaba.com, dwagner@suse.de, tobin@kernel.org, cl@linux.com, akpm@linux-foundation.org, ying.huang@intel.com, dan.j.williams@intel.com, cai@lca.pw, stable@vger.kernel.org Subject: Re: [PATCH 1/3] mm/vmscan: restore zone_reclaim_mode ABI In-Reply-To: <20200701152623.384AF0A7@viggo.jf.intel.com> Message-ID: References: <20200701152621.D520E62B@viggo.jf.intel.com> <20200701152623.384AF0A7@viggo.jf.intel.com> User-Agent: Alpine 2.23 (DEB 453 2020-06-18) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Queue-Id: A5DDC1803F9AD X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 1 Jul 2020, Dave Hansen wrote: > > From: Dave Hansen > > 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 > Fixes: 648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE") > Cc: Ben Widawsky > Cc: Alex Shi > Cc: Daniel Wagner > Cc: "Tobin C. Harding" > Cc: Christoph Lameter > Cc: Andrew Morton > Cc: Huang Ying > Cc: Dan Williams > Cc: Qian Cai > Cc: Daniel Wagner > Cc: stable@vger.kernel.org Acked-by: David Rientjes