Dave Hansen dave.hansen@linux.intel.com writes:
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
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?
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.
Best Regards, Huang, Ying