[RFC PATCH v2 2/2] mm: tuning hardcoded reserved memory

[Andrew Shewmaker]

Add a rootuser_reserve_pages knob to allow admins of large memory 
systems running with overcommit disabled to change the hardcoded 
memory reserve to something other than 3%.

Signed-off-by: Andrew Shewmaker <agshew@xxxxxxxxx>

---

Patch based off of mmotm git tree as of February 27th.

I set rootuser_reserve pages to be a default of 1000, and I suppose 
I should have initialzed similarly to the way min_free_kbytes is, 
scaling it with the size of the box. However, I wanted to get a 
simple version of this patch out for feedback to see if it has any 
chance of acceptance or if I need to take an entirely different 
approach.

Any feedback will be appreciated!

 Documentation/sysctl/vm.txt |  9 +++++++++
 include/linux/mm.h          |  2 ++
 kernel/sysctl.c             |  8 ++++++++
 mm/mmap.c                   | 30 +++++++++++++++++++++++-------
 4 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 078701f..3a71de9 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -51,6 +51,7 @@ Currently, these files are in /proc/sys/vm:
 - page-cluster
 - panic_on_oom
 - percpu_pagelist_fraction
+- rootuser_reserve_pages
 - stat_interval
 - swappiness
 - vfs_cache_pressure
@@ -628,6 +629,14 @@ the high water marks for each per cpu page list.
 
 ==============================================================
 
+rootuser_reserve_pages
+
+The number of free pages left in the system that should be reserved for users
+with the capability cap_sys_admin. The default falue is 3% of total system 
+memory. Changing this takes effect whenever an application requests memory.
+
+==============================================================
+
 stat_interval
 
 The time interval between which vm statistics are updated.  The default
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 66e2f7c..af7b39f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1677,6 +1677,8 @@ int in_gate_area_no_mm(unsigned long addr);
 
 int drop_caches_sysctl_handler(struct ctl_table *, int,
 					void __user *, size_t *, loff_t *);
+int rootuser_reserve_pages_sysctl_handler(struct ctl_table *, int,
+					void __user *, size_t *, loff_t *);
 unsigned long shrink_slab(struct shrink_control *shrink,
 			  unsigned long nr_pages_scanned,
 			  unsigned long lru_pages);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c88878d..cd1987e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -96,6 +96,7 @@
 /* External variables not in a header file. */
 extern int sysctl_overcommit_memory;
 extern int sysctl_overcommit_ratio;
+extern int sysctl_rootuser_reserve_pages;
 extern int max_threads;
 extern int suid_dumpable;
 #ifdef CONFIG_COREDUMP
@@ -1413,6 +1414,13 @@ static struct ctl_table vm_table[] = {
 		.extra2		= &one,
 	},
 #endif
+	{
+		.procname	= "rootuser_reserve_pages",
+		.data		= &sysctl_rootuser_reserve_pages,
+		.maxlen		= sizeof(sysctl_rootuser_reserve_pages),
+		.mode		= 0644,
+		.proc_handler	= rootuser_reserve_pages_sysctl_handler,
+	},
 	{ }
 };
 
diff --git a/mm/mmap.c b/mm/mmap.c
index d1e4124..b58af97 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -32,6 +32,7 @@
 #include <linux/khugepaged.h>
 #include <linux/uprobes.h>
 #include <linux/rbtree_augmented.h>
+#include <linux/sysctl.h>
 
 #include <asm/uaccess.h>
 #include <asm/cacheflush.h>
@@ -83,6 +84,7 @@ EXPORT_SYMBOL(vm_get_page_prot);
 int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;  /* heuristic overcommit */
 int sysctl_overcommit_ratio __read_mostly = 50;	/* default is 50% */
 int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
+int sysctl_rootuser_reserve_pages __read_mostly = 1000;
 /*
  * Make sure vm_committed_as in one cacheline and not cacheline shared with
  * other variables. It can be updated by several CPUs frequently.
@@ -165,7 +167,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 		 * Leave the last 3% for root
 		 */
 		if (!cap_sys_admin)
-			free -= free / 32;
+			free -= sysctl_rootuser_reserve_pages;
 
 		if (free > pages)
 			return 0;
@@ -179,9 +181,9 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 	 * Leave the last 3% for root
 	 */
 	if (!cap_sys_admin)
-		allowed -= allowed / 32;
+		allowed -= sysctl_rootuser_reserve_pages;
 	allowed += total_swap_pages;
 
 	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
 		return 0;
 error:
@@ -3052,3 +3049,22 @@ void __init mmap_init(void)
 	ret = percpu_counter_init(&vm_committed_as, 0);
 	VM_BUG_ON(ret);
 }
+
+/*
+ * rootuser_reserve_pages_sysctl_handler - just a wrapper around proc_dointvec_minmax() so 
+ *	that we can cap the number of pages to the current number of free pages.
+ */
+int rootuser_reserve_pages_sysctl_handler(ctl_table *table, int write, 
+	void __user *buffer, size_t *length, loff_t *ppos)
+{
+	unsigned long free;
+
+	proc_dointvec(table, write, buffer, length, ppos);
+
+	if (write) {
+		free = global_page_state(NR_FREE_PAGES);
+		if (sysctl_rootuser_reserve_pages > free)
+			sysctl_rootuser_reserve_pages = free;
+	}
+	return 0;
+}
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