5 files changed, 47 insertions, 5 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 0b1b0c00861..07231afca72 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -1315,6 +1315,21 @@ for writeout by the pdflush daemons.  It is expressed in 100'ths of a second.
 Data which has been dirty in-memory for longer than this interval will be
 written out next time a pdflush daemon wakes up.
 
+highmem_is_dirtyable
+--------------------
+
+Only present if CONFIG_HIGHMEM is set.
+
+This defaults to 0 (false), meaning that the ratios set above are calculated
+as a percentage of lowmem only.  This protects against excessive scanning
+in page reclaim, swapping and general VM distress.
+
+Setting this to 1 can be useful on 32 bit machines where you want to make
+random changes within an MMAPed file that is larger than your available
+lowmem without causing large quantities of random IO.  Is is safe if the
+behavior of all programs running on the machine is known and memory will
+not be otherwise stressed.
+
 legacy_va_layout
 ----------------
 
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 6f31f0a247d..24eac1bc735 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -22,6 +22,7 @@ Currently, these files are in /proc/sys/vm:
 - dirty_background_ratio
 - dirty_expire_centisecs
 - dirty_writeback_centisecs
+- highmem_is_dirtyable   (only if CONFIG_HIGHMEM set)
 - max_map_count
 - min_free_kbytes
 - laptop_mode
@@ -40,9 +41,9 @@ Currently, these files are in /proc/sys/vm:
 ==============================================================
 
 dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
-dirty_writeback_centisecs, vfs_cache_pressure, laptop_mode,
-block_dump, swap_token_timeout, drop-caches,
-hugepages_treat_as_movable:
+dirty_writeback_centisecs, highmem_is_dirtyable,
+vfs_cache_pressure, laptop_mode, block_dump, swap_token_timeout,
+drop-caches, hugepages_treat_as_movable:
 
 See Documentation/filesystems/proc.txt
 
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index c6148bbf125..b2cd826a8c9 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -100,6 +100,7 @@ extern int dirty_background_ratio;
 extern int vm_dirty_ratio;
 extern int dirty_writeback_interval;
 extern int dirty_expire_interval;
+extern int vm_highmem_is_dirtyable;
 extern int block_dump;
 extern int laptop_mode;
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 7cb1ac3e6ff..d0b47b85906 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -84,8 +84,11 @@ extern int sysctl_stat_interval;
 extern int latencytop_enabled;
 
 /* Constants used for minimum and  maximum */
-#ifdef CONFIG_DETECT_SOFTLOCKUP
+#if defined(CONFIG_DETECT_SOFTLOCKUP) || defined(CONFIG_HIGHMEM)
 static int one = 1;
+#endif
+
+#ifdef CONFIG_DETECT_SOFTLOCKUP
 static int sixty = 60;
 #endif
 
@@ -1150,6 +1153,19 @@ static struct ctl_table vm_table[] = {
                 .extra1         = &zero,
         },
 #endif
+#ifdef CONFIG_HIGHMEM
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "highmem_is_dirtyable",
+                .data           = &vm_highmem_is_dirtyable,
+                .maxlen         = sizeof(vm_highmem_is_dirtyable),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec_minmax,
+                .strategy       = &sysctl_intvec,
+                .extra1         = &zero,
+                .extra2         = &one,
+        },
+#endif
 /*
  * NOTE: do not add new entries to this table unless you have read
  * Documentation/sysctl/ctl_unnumbered.txt
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 8137482abd6..c689b60af00 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -69,6 +69,12 @@ static inline long sync_writeback_pages(void)
 int dirty_background_ratio = 5;
 
 /*
+ * free highmem will not be subtracted from the total free memory
+ * for calculating free ratios if vm_highmem_is_dirtyable is true
+ */
+int vm_highmem_is_dirtyable;
+
+/*
  * The generator of dirty data starts writeback at this percentage
  */
 int vm_dirty_ratio = 10;
@@ -287,7 +293,10 @@ static unsigned long determine_dirtyable_memory(void)
         x = global_page_state(NR_FREE_PAGES)
                 + global_page_state(NR_INACTIVE)
                 + global_page_state(NR_ACTIVE);
-        x -= highmem_dirtyable_memory(x);
+
+        if (!vm_highmem_is_dirtyable)
+                x -= highmem_dirtyable_memory(x);
+
         return x + 1;   /* Ensure that we never return 0 */
 }