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Re: [patch 1/4 v2] mm: exclude reserved pages from dirtyable memory

To: Johannes Weiner <jweiner@xxxxxxxxxx>
Subject: Re: [patch 1/4 v2] mm: exclude reserved pages from dirtyable memory
From: Minchan Kim <minchan.kim@xxxxxxxxx>
Date: Wed, 28 Sep 2011 13:55:51 +0900
Cc: Mel Gorman <mgorman@xxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Chris Mason <chris.mason@xxxxxxxxxx>, "Theodore Ts'o" <tytso@xxxxxxx>, Andreas Dilger <adilger.kernel@xxxxxxxxx>, xfs@xxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
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Hi Hannes,

On Fri, Sep 23, 2011 at 04:38:17PM +0200, 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>
> ---
>  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 b156e80..9021453 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.
> +                      */

Could you put Mel's description instead of it if you don't mind?
If I didn't see Mel's thing, maybe I wouldn't suggest but it looks
more easier to understand.

> +                     zone->dirty_balance_reserve = max;
>               }
>       }
> +     dirty_balance_reserve = reserve_pages;
>       totalreserve_pages = reserve_pages;
>  }
>  
> -- 
> 1.7.6.2
> 

-- 
Kinds regards,
Minchan Kim

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