mm: honor min_free_kbytes set by user

min_free_kbytes is updated during memory hotplug (by
init_per_zone_wmark_min) currently which is right thing to do in most
cases but this could be unexpected if admin increased the value to
prevent from allocation failures and the new min_free_kbytes would be
decreased as a result of memory hotadd.

This patch saves the user defined value and allows updating
min_free_kbytes only if it is higher than the saved one.

A warning is printed when the new value is ignored.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 7 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b5855e545eec..b100255dedda 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -204,6 +204,7 @@ static char * const zone_names[MAX_NR_ZONES] = {
 };
 
 int min_free_kbytes = 1024;
+int user_min_free_kbytes;
 
 static unsigned long __meminitdata nr_kernel_pages;
 static unsigned long __meminitdata nr_all_pages;
@@ -5589,14 +5590,21 @@ static void __meminit setup_per_zone_inactive_ratio(void)
 int __meminit init_per_zone_wmark_min(void)
 {
         unsigned long lowmem_kbytes;
+        int new_min_free_kbytes;
 
         lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
-
-        min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
-        if (min_free_kbytes < 128)
-                min_free_kbytes = 128;
-        if (min_free_kbytes > 65536)
-                min_free_kbytes = 65536;
+        new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
+
+        if (new_min_free_kbytes > user_min_free_kbytes) {
+                min_free_kbytes = new_min_free_kbytes;
+                if (min_free_kbytes < 128)
+                        min_free_kbytes = 128;
+                if (min_free_kbytes > 65536)
+                        min_free_kbytes = 65536;
+        } else {
+                pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
+                                new_min_free_kbytes, user_min_free_kbytes);
+        }
         setup_per_zone_wmarks();
         refresh_zone_stat_thresholds();
         setup_per_zone_lowmem_reserve();
@@ -5614,8 +5622,10 @@ int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
         void __user *buffer, size_t *length, loff_t *ppos)
 {
         proc_dointvec(table, write, buffer, length, ppos);
-        if (write)
+        if (write) {
+                user_min_free_kbytes = min_free_kbytes;
                 setup_per_zone_wmarks();
+        }
         return 0;
 }