mm: replace hardcoded 3% with admin_reserve_pages knob

Add an admin_reserve_kbytes knob to allow admins to change the hardcoded
memory reserve to something other than 3%, which may be multiple
gigabytes on large memory systems.  Only about 8MB is necessary to
enable recovery in the default mode, and only a few hundred MB are
required even when overcommit is disabled.

This affects OVERCOMMIT_GUESS and OVERCOMMIT_NEVER.

admin_reserve_kbytes is initialized to min(3% free pages, 8MB)

I arrived at 8MB by summing the RSS of sshd or login, bash, and top.

Please see first patch in this series for full background, motivation,
testing, and full changelog.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: make init_admin_reserve() static]
Signed-off-by: Andrew Shewmaker <agshew@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6daabb72bdb5..9edcf456e0fc 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1436,6 +1436,13 @@ static struct ctl_table vm_table[] = {
                 .mode           = 0644,
                 .proc_handler   = proc_doulongvec_minmax,
         },
+        {
+                .procname       = "admin_reserve_kbytes",
+                .data           = &sysctl_admin_reserve_kbytes,
+                .maxlen         = sizeof(sysctl_admin_reserve_kbytes),
+                .mode           = 0644,
+                .proc_handler   = proc_doulongvec_minmax,
+        },
         { }
 };