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Date:   Wed, 1 Jul 2020 13:04:46 -0700
From:   Ben Widawsky <ben.widawsky@intel.com>
To:     David Rientjes <rientjes@google.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        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
Subject: Re: [PATCH 3/3] mm/vmscan: replace implicit RECLAIM_ZONE checks with
 explicit checks
Message-ID: <20200701200446.ki5hdgarcpmo2vuf@intel.com>
Mail-Followup-To: David Rientjes <rientjes@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        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
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On 20-07-01 13:03:01, David Rientjes wrote:
> On Wed, 1 Jul 2020, Dave Hansen wrote:
> 
> > diff -puN include/linux/swap.h~mm-vmscan-node_reclaim_mode_helper include/linux/swap.h
> > --- a/include/linux/swap.h~mm-vmscan-node_reclaim_mode_helper	2020-07-01 08:22:13.650955330 -0700
> > +++ b/include/linux/swap.h	2020-07-01 08:22:13.659955330 -0700
> > @@ -12,6 +12,7 @@
> >  #include <linux/fs.h>
> >  #include <linux/atomic.h>
> >  #include <linux/page-flags.h>
> > +#include <uapi/linux/mempolicy.h>
> >  #include <asm/page.h>
> >  
> >  struct notifier_block;
> > @@ -374,6 +375,12 @@ extern int sysctl_min_slab_ratio;
> >  #define node_reclaim_mode 0
> >  #endif
> >  
> > +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?
> 

I don't think so because I don't think anything else validates the unused bits
remain unused.