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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