Overcommit (OOM) problem on embedded device (PPChameleon)

[David Adair]

> > Look in mm/mmap.c and search for overcommit_memory, then do same
> > in the sources for your redhat kernels.
> I'll look into RH kernel to see if it isn't like you say :-)
> 
> > Perhaps there's a patch for it floating around somewhere? ;-)
> Now, that would be really nice.
> Though I have no idea of where to look!?
> 

Here is what I do.  Patch is against BK linuxppc-2.4 version 1.1285
(2.4.28-pre3) but might work.

Burns 5% of the RAM but alternatives all seem worse and I never
have much luck trying to convince my colleagues that
dynamic allocation has no place in an embedded system.

If you find out why RH works it would be interesting ... perhaps
there is a better way.

David


#
# mm/mmap.c
#   Add pessimistic overcommit mode similar to 2.6 mode 2.
#   This allows malloc aka sbrk() to actually fail before
#   process is killed.
#   Overloaded sysctl_overcommit_memory to be both enable
#   and ratio to avoid making a new sysctl.
# 
diff -Nru a/mm/mmap.c b/mm/mmap.c
--- a/mm/mmap.c	2005-03-30 07:15:13 -08:00
+++ b/mm/mmap.c	2005-03-30 07:15:13 -08:00
@@ -45,9 +45,13 @@
 	__S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
 };
 
-int sysctl_overcommit_memory;
+int sysctl_overcommit_memory = 98;
+
 int max_map_count = DEFAULT_MAX_MAP_COUNT;
 
+extern unsigned long totalram_pages;
+extern unsigned long totalhigh_pages;
+
 /* Check that a process has enough memory to allocate a
  * new virtual mapping.
  */
@@ -66,7 +70,7 @@
 	unsigned long free;
 	
         /* Sometimes we want to use more memory than we have. */
-	if (sysctl_overcommit_memory)
+	if (sysctl_overcommit_memory == 1)
 	    return 1;
 
 	/* The page cache contains buffer pages these days.. */
@@ -91,7 +95,20 @@
 	free += (dentry_stat.nr_unused * sizeof(struct dentry)) >>
PAGE_SHIFT;
 	free += (inodes_stat.nr_unused * sizeof(struct inode)) >>
PAGE_SHIFT;
 
+	/*
+	 * Leave the last 3% for root
+	 */
+	if (current->euid)
+		free -= free / 32;
+
+	/* Strict mode do not allocate last bit of memory */
+	if (sysctl_overcommit_memory) {
+		pages += (totalram_pages - totalhigh_pages)
+			* (100 - sysctl_overcommit_memory) / 100;
+	}
+
 	return free > pages;
+
 }
 
 /* Remove one vm structure from the inode's i_mapping address space. */