[PATCH 1/2] mm: page_alloc: generalize the dirty balance reserve

[PATCH 1/2] mm: page_alloc: generalize the dirty balance reserve

[Johannes Weiner]

The dirty balance reserve that dirty throttling has to consider is
merely memory not available to userspace allocations. There is nothing
writeback-specific about it. Generalize the name so that it's reusable
outside of that context.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/mmzone.h |  6 +++---
 include/linux/swap.h   |  1 -
 mm/page-writeback.c    | 14 ++++++++++++--
 mm/page_alloc.c        | 21 +++------------------
 4 files changed, 18 insertions(+), 24 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index e23a9e7..9134ae3 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -361,10 +361,10 @@ struct zone {
 	struct per_cpu_pageset __percpu *pageset;
 
 	/*
-	 * This is a per-zone reserve of pages that should not be
-	 * considered dirtyable memory.
+	 * This is a per-zone reserve of pages that are not available
+	 * to userspace allocations.
 	 */
-	unsigned long		dirty_balance_reserve;
+	unsigned long		totalreserve_pages;
 
 #ifndef CONFIG_SPARSEMEM
 	/*
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7ba7dcc..066bd21 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -287,7 +287,6 @@ static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
 /* linux/mm/page_alloc.c */
 extern unsigned long totalram_pages;
 extern unsigned long totalreserve_pages;
-extern unsigned long dirty_balance_reserve;
 extern unsigned long nr_free_buffer_pages(void);
 extern unsigned long nr_free_pagecache_pages(void);
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2c90357..8e5f2fd 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -278,7 +278,12 @@ static unsigned long zone_dirtyable_memory(struct zone *zone)
 	unsigned long nr_pages;
 
 	nr_pages = zone_page_state(zone, NR_FREE_PAGES);
-	nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	nr_pages -= min(nr_pages, zone->totalreserve_pages);
 
 	nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
 	nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
@@ -332,7 +337,12 @@ static unsigned long global_dirtyable_memory(void)
 	unsigned long x;
 
 	x = global_page_state(NR_FREE_PAGES);
-	x -= min(x, dirty_balance_reserve);
+	/*
+	 * Pages reserved for the kernel should not be considered
+	 * dirtyable, to prevent a situation where reclaim has to
+	 * clean pages in order to balance the zones.
+	 */
+	x -= min(x, totalreserve_pages);
 
 	x += global_page_state(NR_INACTIVE_FILE);
 	x += global_page_state(NR_ACTIVE_FILE);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 17a3c66..e786d7e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -114,13 +114,6 @@ static DEFINE_SPINLOCK(managed_page_count_lock);
 unsigned long totalram_pages __read_mostly;
 unsigned long totalreserve_pages __read_mostly;
 unsigned long totalcma_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;
@@ -5955,20 +5948,12 @@ static void calculate_totalreserve_pages(void)
 
 			if (max > zone->managed_pages)
 				max = zone->managed_pages;
+
+			zone->totalreserve_pages = max;
+
 			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;
 }
 
-- 
2.6.2

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[PATCH 2/2] proc: meminfo: estimate available memory more conservatively

[Johannes Weiner]

The MemAvailable item in /proc/meminfo is to give users a hint of how
much memory is allocatable without causing swapping, so it excludes
the zones' low watermarks as unavailable to userspace.

However, for a userspace allocation, kswapd will actually reclaim
until the free pages hit a combination of the high watermark and the
page allocator's lowmem protection that keeps a certain amount of DMA
and DMA32 memory from userspace as well.

Subtract the full amount we know to be unavailable to userspace from
the number of free pages when calculating MemAvailable.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 fs/proc/meminfo.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 9155a5a..df4661a 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -57,11 +57,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	/*
 	 * Estimate the amount of memory available for userspace allocations,
 	 * without causing swapping.
-	 *
-	 * Free memory cannot be taken below the low watermark, before the
-	 * system starts swapping.
 	 */
-	available = i.freeram - wmark_low;
+	available = i.freeram - totalreserve_pages;
 
 	/*
 	 * Not all the page cache can be freed, otherwise the system will
-- 
2.6.2

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Re: [PATCH 2/2] proc: meminfo: estimate available memory more conservatively

[Michal Hocko]

On Mon 30-11-15 15:00:22, Johannes Weiner wrote:
> The MemAvailable item in /proc/meminfo is to give users a hint of how
> much memory is allocatable without causing swapping, so it excludes
> the zones' low watermarks as unavailable to userspace.
> 
> However, for a userspace allocation, kswapd will actually reclaim
> until the free pages hit a combination of the high watermark and the
> page allocator's lowmem protection that keeps a certain amount of DMA
> and DMA32 memory from userspace as well.
> 
> Subtract the full amount we know to be unavailable to userspace from
> the number of free pages when calculating MemAvailable.

I am not sure this will make a big or even noticeable difference in the
real life but it makes sense.

> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  fs/proc/meminfo.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 9155a5a..df4661a 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -57,11 +57,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	/*
>  	 * Estimate the amount of memory available for userspace allocations,
>  	 * without causing swapping.
> -	 *
> -	 * Free memory cannot be taken below the low watermark, before the
> -	 * system starts swapping.
>  	 */
> -	available = i.freeram - wmark_low;
> +	available = i.freeram - totalreserve_pages;
>  
>  	/*
>  	 * Not all the page cache can be freed, otherwise the system will
> -- 
> 2.6.2
> 
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-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/2] mm: page_alloc: generalize the dirty balance reserve

[Michal Hocko]

On Mon 30-11-15 15:00:21, Johannes Weiner wrote:
> The dirty balance reserve that dirty throttling has to consider is
> merely memory not available to userspace allocations. There is nothing
> writeback-specific about it. Generalize the name so that it's reusable
> outside of that context.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/mmzone.h |  6 +++---
>  include/linux/swap.h   |  1 -
>  mm/page-writeback.c    | 14 ++++++++++++--
>  mm/page_alloc.c        | 21 +++------------------
>  4 files changed, 18 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e23a9e7..9134ae3 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -361,10 +361,10 @@ struct zone {
>  	struct per_cpu_pageset __percpu *pageset;
>  
>  	/*
> -	 * This is a per-zone reserve of pages that should not be
> -	 * considered dirtyable memory.
> +	 * This is a per-zone reserve of pages that are not available
> +	 * to userspace allocations.
>  	 */
> -	unsigned long		dirty_balance_reserve;
> +	unsigned long		totalreserve_pages;
>  
>  #ifndef CONFIG_SPARSEMEM
>  	/*
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7ba7dcc..066bd21 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -287,7 +287,6 @@ static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
>  /* linux/mm/page_alloc.c */
>  extern unsigned long totalram_pages;
>  extern unsigned long totalreserve_pages;
> -extern unsigned long dirty_balance_reserve;
>  extern unsigned long nr_free_buffer_pages(void);
>  extern unsigned long nr_free_pagecache_pages(void);
>  
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2c90357..8e5f2fd 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -278,7 +278,12 @@ static unsigned long zone_dirtyable_memory(struct zone *zone)
>  	unsigned long nr_pages;
>  
>  	nr_pages = zone_page_state(zone, NR_FREE_PAGES);
> -	nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
> +	/*
> +	 * Pages reserved for the kernel should not be considered
> +	 * dirtyable, to prevent a situation where reclaim has to
> +	 * clean pages in order to balance the zones.
> +	 */
> +	nr_pages -= min(nr_pages, zone->totalreserve_pages);
>  
>  	nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
>  	nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
> @@ -332,7 +337,12 @@ static unsigned long global_dirtyable_memory(void)
>  	unsigned long x;
>  
>  	x = global_page_state(NR_FREE_PAGES);
> -	x -= min(x, dirty_balance_reserve);
> +	/*
> +	 * Pages reserved for the kernel should not be considered
> +	 * dirtyable, to prevent a situation where reclaim has to
> +	 * clean pages in order to balance the zones.
> +	 */
> +	x -= min(x, totalreserve_pages);
>  
>  	x += global_page_state(NR_INACTIVE_FILE);
>  	x += global_page_state(NR_ACTIVE_FILE);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 17a3c66..e786d7e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -114,13 +114,6 @@ static DEFINE_SPINLOCK(managed_page_count_lock);
>  unsigned long totalram_pages __read_mostly;
>  unsigned long totalreserve_pages __read_mostly;
>  unsigned long totalcma_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;
> @@ -5955,20 +5948,12 @@ static void calculate_totalreserve_pages(void)
>  
>  			if (max > zone->managed_pages)
>  				max = zone->managed_pages;
> +
> +			zone->totalreserve_pages = max;
> +
>  			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;
>  }
>  
> -- 
> 2.6.2
> 
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-- 
Michal Hocko
SUSE Labs

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