Overcommit (OOM) problem on embedded device (PPChameleon)

[Martin Egholm Nielsen]

Helloooo David!

>>>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.
Uhhuuuuuuu!! It works!!!

With my simple exhaust_mem example it works with a value of 98 (that is 
5% of waste).
However, I need setting it to 95 (so ~8% waste) in order to make it work 
entirely with my other application... But that's better than letting the 
OOM take over control.

> 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.
I'll let you know if I find the triggering patch in RH's kernels :-)

THANKS,
  Martin Egholm

> 
> 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. */
> 
> 
>