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Date:   Wed, 1 Jul 2020 13:03:01 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>
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
Subject: Re: [PATCH 3/3] mm/vmscan: replace implicit RECLAIM_ZONE checks with
 explicit checks
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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?