Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> writes:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> 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
