Re: [PATCH V6] Allow compaction of unevictable pages

[Michal Hocko]

On Thu 19-03-15 09:57:10, Eric B Munson wrote:
> Currently, pages which are marked as unevictable are protected from
> compaction, but not from other types of migration.  The POSIX real time
> extension explicitly states that mlock() will prevent a major page
> fault, but the spirit of is is that mlock() should give a process the
> ability to control sources of latency, including minor page faults.
> However, the mlock manpage only explicitly says that a locked page will
> not be written to swap and this can cause some confusion.  The
> compaction code today, does not give a developer who wants to avoid swap
> but wants to have large contiguous areas available any method to achieve
> this state.  This patch introduces a sysctl for controlling compaction
> behavoir with respect to the unevictable lru.  Users that demand no page
> faults after a page is present can set compact_unevictable to 0 and
> users who need the large contiguous areas can enable compaction on
> locked memory by leaving the default value of 1.
> 
> To illustrate this problem I wrote a quick test program that mmaps a
> large number of 1MB files filled with random data.  These maps are
> created locked and read only.  Then every other mmap is unmapped and I
> attempt to allocate huge pages to the static huge page pool.  When the
> compact_unevictable sysctl is 0, I cannot allocate hugepages after
> fragmenting memory.  When the value is set to 1, allocations succeed.
> 
> Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: linux-rt-users@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-api@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Michal Hocko <mhocko@xxxxxxx>
Thanks!

> ---
> Changes from V5:
> * Default sysctl value is now 1
> * Used more descriptive sysctl name
> * documentation calss out default value
> 
>  Documentation/sysctl/vm.txt |   11 +++++++++++
>  include/linux/compaction.h  |    1 +
>  kernel/sysctl.c             |    9 +++++++++
>  mm/compaction.c             |    7 +++++++
>  4 files changed, 28 insertions(+)
> 
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index 902b457..9832ec5 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -21,6 +21,7 @@ Currently, these files are in /proc/sys/vm:
>  - admin_reserve_kbytes
>  - block_dump
>  - compact_memory
> +- compact_unevictable_allowed
>  - dirty_background_bytes
>  - dirty_background_ratio
>  - dirty_bytes
> @@ -106,6 +107,16 @@ huge pages although processes will also directly compact memory as required.
>  
>  ==============================================================
>  
> +compact_unevictable_allowed
> +
> +Available only when CONFIG_COMPACTION is set. When set to 1, compaction is
> +allowed to examine the unevictable lru (mlocked pages) for pages to compact.
> +This should be used on systems where stalls for minor page faults are an
> +acceptable trade for large contiguous free memory.  Set to 0 to prevent
> +compaction from moving pages that are unevictable.  Default value is 1.
> +
> +==============================================================
> +
>  dirty_background_bytes
>  
>  Contains the amount of dirty memory at which the background kernel
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index a014559..aa8f61c 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -34,6 +34,7 @@ extern int sysctl_compaction_handler(struct ctl_table *table, int write,
>  extern int sysctl_extfrag_threshold;
>  extern int sysctl_extfrag_handler(struct ctl_table *table, int write,
>  			void __user *buffer, size_t *length, loff_t *ppos);
> +extern int sysctl_compact_unevictable_allowed;
>  
>  extern int fragmentation_index(struct zone *zone, unsigned int order);
>  extern unsigned long try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 88ea2d6..2f6c880 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1313,6 +1313,15 @@ static struct ctl_table vm_table[] = {
>  		.extra1		= &min_extfrag_threshold,
>  		.extra2		= &max_extfrag_threshold,
>  	},
> +	{
> +		.procname	= "compact_unevictable_allowed",
> +		.data		= &sysctl_compact_unevictable_allowed,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +		.extra1		= &zero,
> +		.extra2		= &one,
> +	},
>  
>  #endif /* CONFIG_COMPACTION */
>  	{
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8c0d945..ad88a8c 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1047,6 +1047,12 @@ typedef enum {
>  } isolate_migrate_t;
>  
>  /*
> + * Allow userspace to control policy on scanning the unevictable LRU for
> + * compactable pages.
> + */
> +int sysctl_compact_unevictable_allowed __read_mostly = 1;
> +
> +/*
>   * Isolate all pages that can be migrated from the first suitable block,
>   * starting at the block pointed to by the migrate scanner pfn within
>   * compact_control.
> @@ -1057,6 +1063,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>  	unsigned long low_pfn, end_pfn;
>  	struct page *page;
>  	const isolate_mode_t isolate_mode =
> +		(sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
>  		(cc->mode == MIGRATE_ASYNC ? ISOLATE_ASYNC_MIGRATE : 0);
>  
>  	/*
> -- 
> 1.7.9.5
> 

-- 
Michal Hocko
SUSE Labs
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