path: root/kernel
diff options
authorEric B Munson <>2015-04-15 16:13:20 -0700
committerLinus Torvalds <>2015-04-15 16:35:17 -0700
commit5bbe3547aa3ba5242366a322a28996872301b703 (patch)
treefca743e8713bc9b5d807083df50fdc120314fdc4 /kernel
parenta4bb3ecdc12a78dc4d0e690d40ec10887b640786 (diff)
mm: allow compaction of unevictable pages
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 this 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 behavior with respect to the unevictable lru. Users who demand no page faults after a page is present can set compact_unevictable_allowed 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_allowed 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 <> Acked-by: Michal Hocko <> Acked-by: Vlastimil Babka <> Acked-by: Christoph Lameter <> Acked-by: David Rientjes <> Acked-by: Rik van Riel <> Cc: Vlastimil Babka <> Cc: Thomas Gleixner <> Cc: Christoph Lameter <> Cc: Peter Zijlstra <> Cc: Mel Gorman <> Cc: David Rientjes <> Cc: Michal Hocko <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'kernel')
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8c0eabd41886..42b7fc2860c1 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1335,6 +1335,15 @@ 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,
+ },