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, 26 insertions, 4 deletions

diff --git a/mm/nommu.c b/mm/nommu.c
index 58e4a0a5125f..fbe3e2f317eb 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -64,6 +64,7 @@ int sysctl_overcommit_ratio = 50; /* default is 50% */
 int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
 int sysctl_nr_trim_pages = CONFIG_NOMMU_INITIAL_TRIM_EXCESS;
 unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */
+unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */
 int heap_stack_gap = 0;
 
 atomic_long_t mmap_pages_allocated;
@@ -1939,10 +1940,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
                         free -= totalreserve_pages;
 
                 /*
-                 * Leave the last 3% for root
+                 * Reserve some for root
                  */
                 if (!cap_sys_admin)
-                        free -= free / 32;
+                        free -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
 
                 if (free > pages)
                         return 0;
@@ -1952,10 +1953,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 
         allowed = totalram_pages * sysctl_overcommit_ratio / 100;
         /*
-         * Leave the last 3% for root
+         * Reserve some 3% for root
          */
         if (!cap_sys_admin)
-                allowed -= allowed / 32;
+                allowed -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
         allowed += total_swap_pages;
 
         /*
@@ -2147,3 +2148,24 @@ static int __meminit init_user_reserve(void)
         return 0;
 }
 module_init(init_user_reserve)
+
+/*
+ * Initialise sysctl_admin_reserve_kbytes.
+ *
+ * The purpose of sysctl_admin_reserve_kbytes is to allow the sys admin
+ * to log in and kill a memory hogging process.
+ *
+ * Systems with more than 256MB will reserve 8MB, enough to recover
+ * with sshd, bash, and top in OVERCOMMIT_GUESS. Smaller systems will
+ * only reserve 3% of free pages by default.
+ */
+static int __meminit init_admin_reserve(void)
+{
+        unsigned long free_kbytes;
+
+        free_kbytes = global_page_state(NR_FREE_PAGES) << (PAGE_SHIFT - 10);
+
+        sysctl_admin_reserve_kbytes = min(free_kbytes / 32, 1UL << 13);
+        return 0;
+}
+module_init(init_admin_reserve)