Re: [PATCH 3/3] mm/vmscan: replace implicit RECLAIM_ZONE checks with explicit checks

[David Rientjes <rientjes@google.com>]

On Wed, 1 Jul 2020, Dave Hansen wrote:

> On 7/1/20 1:04 PM, Ben Widawsky wrote:
> >> +static inline bool node_reclaim_enabled(void)
> >> +{
> >> +	/* Is any node_reclaim_mode bit set? */
> >> +	return node_reclaim_mode & (RECLAIM_ZONE|RECLAIM_WRITE|RECLAIM_UNMAP);
> >> +}
> >> +
> >>  extern void check_move_unevictable_pages(struct pagevec *pvec);
> >>  
> >>  extern int kswapd_run(int nid);
> > If a user writes a bit that isn't a RECLAIM_* bit to vm.zone_reclaim_mode 
> > today, it acts as though RECLAIM_ZONE is enabled: we try to reclaim in 
> > zonelist order before falling back to the next zone in the page allocator.  
> > The sysctl doesn't enforce any max value :/  I dont know if there is any 
> > such user, but this would break them if there is.
> > 
> > Should this simply be return !!node_reclaim_mode?
> 
> You're right that there _could_ be a user-visible behavior change here.
>  But, if there were a change it would be for a bit which wasn't even
> mentioned in the documentation.  Somebody would have had to look at the
> doc mentioning 1,2,4 and written an 8.  If they did that, they're asking
> for trouble because we could have defined the '8' bit to do nasty things
> like auto-demote all your memory. :)
> 
> I'll mention it in the changelog, but I still think we should check the
> actual, known bits rather than check for 0.
> 
> BTW, in the hardware, they almost invariably make unused bits "reserved"
> and do mean things like #GP if someone tries to set them.  This is a
> case where the kernel probably should have done the same.  It would have
> saved us the trouble of asking these questions now.  Maybe we should
> even do that going forward.
> 

Maybe enforce it in a sysctl handler so the user catches any errors, which 
would be better than silently accepting some policy that doesn't exist?

RECLAIM_UNMAP and/or RECLAIM_WRITE should likely get -EINVAL if attempted 
to be set without RECLAIM_ZONE as well: they are no-ops without 
RECLAIM_ZONE.  This would likely have caught something wrong with commit 
648b5cf368e0 ("mm/vmscan: remove unused RECLAIM_OFF/RECLAIM_ZONE") if it 
would have already been in place.

I don't feel strongly about this, so feel free to ignore.