[PATCH V6] Allow compaction of unevictable pages
From: Eric B Munson <hidden>
Date: 2015-03-19 13:57:25
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 <redacted> Cc: Vlastimil Babka <redacted> Cc: Thomas Gleixner <redacted> Cc: Christoph Lameter <redacted> Cc: Peter Zijlstra <redacted> Cc: Mel Gorman <redacted> Cc: David Rientjes <redacted> Cc: Rik van Riel <redacted> Cc: Michal Hocko <redacted> Cc: linux-rt-users-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org Cc: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org --- 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); /*
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1.7.9.5