Re: [patch 1/5] mm: exclude reserved pages from dirtyable memory

[Michal Hocko]

On Fri 30-09-11 09:17:20, Johannes Weiner wrote:
> The amount of dirtyable pages should not include the full number of
> free pages: there is a number of reserved pages that the page
> allocator and kswapd always try to keep free.
> 
> The closer (reclaimable pages - dirty pages) is to the number of
> reserved pages, the more likely it becomes for reclaim to run into
> dirty pages:
> 
>        +----------+ ---
>        |   anon   |  |
>        +----------+  |
>        |          |  |
>        |          |  -- dirty limit new    -- flusher new
>        |   file   |  |                     |
>        |          |  |                     |
>        |          |  -- dirty limit old    -- flusher old
>        |          |                        |
>        +----------+                       --- reclaim
>        | reserved |
>        +----------+
>        |  kernel  |
>        +----------+
> 
> This patch introduces a per-zone dirty reserve that takes both the
> lowmem reserve as well as the high watermark of the zone into account,
> and a global sum of those per-zone values that is subtracted from the
> global amount of dirtyable pages.  The lowmem reserve is unavailable
> to page cache allocations and kswapd tries to keep the high watermark
> free.  We don't want to end up in a situation where reclaim has to
> clean pages in order to balance zones.
> 
> Not treating reserved pages as dirtyable on a global level is only a
> conceptual fix.  In reality, dirty pages are not distributed equally
> across zones and reclaim runs into dirty pages on a regular basis.
> 
> But it is important to get this right before tackling the problem on a
> per-zone level, where the distance between reclaim and the dirty pages
> is mostly much smaller in absolute numbers.
> 
> Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>

Makes sense.
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  include/linux/mmzone.h |    6 ++++++
>  include/linux/swap.h   |    1 +
>  mm/page-writeback.c    |    6 ++++--
>  mm/page_alloc.c        |   19 +++++++++++++++++++
>  4 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1ed4116..37a61e7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -317,6 +317,12 @@ struct zone {
>  	 */
>  	unsigned long		lowmem_reserve[MAX_NR_ZONES];
>  
> +	/*
> +	 * This is a per-zone reserve of pages that should not be
> +	 * considered dirtyable memory.
> +	 */
> +	unsigned long		dirty_balance_reserve;
> +
>  #ifdef CONFIG_NUMA
>  	int node;
>  	/*
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 3808f10..5e70f65 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -209,6 +209,7 @@ struct swap_list_t {
>  /* linux/mm/page_alloc.c */
>  extern unsigned long totalram_pages;
>  extern unsigned long totalreserve_pages;
> +extern unsigned long dirty_balance_reserve;
>  extern unsigned int nr_free_buffer_pages(void);
>  extern unsigned int nr_free_pagecache_pages(void);
>  
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index da6d263..c8acf8a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -170,7 +170,8 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
>  			&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
>  
>  		x += zone_page_state(z, NR_FREE_PAGES) +
> -		     zone_reclaimable_pages(z);
> +		     zone_reclaimable_pages(z) -
> +		     zone->dirty_balance_reserve;
>  	}
>  	/*
>  	 * Make sure that the number of highmem pages is never larger
> @@ -194,7 +195,8 @@ static unsigned long determine_dirtyable_memory(void)
>  {
>  	unsigned long x;
>  
> -	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> +	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
> +	    dirty_balance_reserve;
>  
>  	if (!vm_highmem_is_dirtyable)
>  		x -= highmem_dirtyable_memory(x);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1dba05e..f8cba89 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -96,6 +96,14 @@ EXPORT_SYMBOL(node_states);
>  
>  unsigned long totalram_pages __read_mostly;
>  unsigned long totalreserve_pages __read_mostly;
> +/*
> + * When calculating the number of globally allowed dirty pages, there
> + * is a certain number of per-zone reserves that should not be
> + * considered dirtyable memory.  This is the sum of those reserves
> + * over all existing zones that contribute dirtyable memory.
> + */
> +unsigned long dirty_balance_reserve __read_mostly;
> +
>  int percpu_pagelist_fraction;
>  gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
>  
> @@ -5076,8 +5084,19 @@ static void calculate_totalreserve_pages(void)
>  			if (max > zone->present_pages)
>  				max = zone->present_pages;
>  			reserve_pages += max;
> +			/*
> +			 * Lowmem reserves are not available to
> +			 * GFP_HIGHUSER page cache allocations and
> +			 * kswapd tries to balance zones to their high
> +			 * watermark.  As a result, neither should be
> +			 * regarded as dirtyable memory, to prevent a
> +			 * situation where reclaim has to clean pages
> +			 * in order to balance the zones.
> +			 */
> +			zone->dirty_balance_reserve = max;
>  		}
>  	}
> +	dirty_balance_reserve = reserve_pages;
>  	totalreserve_pages = reserve_pages;
>  }
>  
> -- 
> 1.7.6.2
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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