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SubjectRe: [PATCH] Prevent OOM from killing init
>Right now my best approximation is to make the OOM test be as optimistic as
>it is safe to be, and the vm_enough_memory() test as pessimistic as
>sensible. Expect a test patch to appear on this list soon.

...and here it is!

This fixes a number of small but linked problems:

- malloc() never returned 0 when the system ran out of memory, instead the OOM killer was triggered. Now, malloc() will return 0 if the calling process is more than 4 times the size of the amount of free memory. As a speedup, available swap space is not considered unless physical memory is not sufficient to contain the process. Note that if overcommit_memory is switched on, malloc() will never return 0 anyway.

- OOM killer was triggered too early - now takes account of buffer and cache memory, which can be cannibalised before the system has completely run out.

- OOM killer badness() factors readjusted in favour of Oracle-like processes (consuming 10's of MB of RAM but up for 3 days or so and with a low-order UID? Now less likely to be killed...)

--- begin oom-patch.diff ---
diff -u linux-2.4.1.orig/mm/mmap.c linux/mm/mmap.c
--- linux-2.4.1.orig/mm/mmap.c Mon Jan 29 16:10:41 2001
+++ linux/mm/mmap.c Sat Mar 24 19:29:51 2001
@@ -54,6 +54,7 @@
*/

long free;
+ struct sysinfo swp_info;

/* Sometimes we want to use more memory than we have. */
if (sysctl_overcommit_memory)
@@ -62,8 +63,32 @@
free = atomic_read(&buffermem_pages);
free += atomic_read(&page_cache_size);
free += nr_free_pages();
- free += nr_swap_pages;
- return free > pages;
+
+ /* Attempt to curtail memory allocations before hard OOM occurs.
+ * Based on current process size, which is hopefully a good and fast heuristic.
+ * Also fix bug where the real OOM limit of (free == freepages.min) is not taken into account.
+ * In fact, we use freepages.high as the threshold to make sure there's still room for buffers+cache.
+ *
+ * -- Jonathan "Chromatix" Morton, 24th March 2001
+ */
+
+ if(current && current->mm)
+ free -= (current->mm->total_vm / 4);
+
+ free -= freepages.high;
+
+ /* Since getting swap info is expensive, see if our allocation can happen in physical RAM */
+ if(free > pages)
+ return 1;
+
+ /* Use the number of FREE swap pages, not the total */
+ si_swapinfo(&swp_info);
+ free += swp_info.freeswap;
+
+ if(free > pages)
+ return 1;
+
+ return 0;
}

/* Remove one vm structure from the inode's i_mapping address space. */
Only in linux/mm/: mmap.c~
diff -u linux-2.4.1.orig/mm/oom_kill.c linux/mm/oom_kill.c
--- linux-2.4.1.orig/mm/oom_kill.c Tue Nov 14 18:56:46 2000
+++ linux/mm/oom_kill.c Sat Mar 24 20:35:20 2001
@@ -76,7 +76,9 @@
run_time = (jiffies - p->start_time) >> (SHIFT_HZ + 10);

points /= int_sqrt(cpu_time);
- points /= int_sqrt(int_sqrt(run_time));
+
+ /* Long-running processes are *very* important, so don't take the 4th root */
+ points /= run_time;

/*
* Niced processes are most likely less important, so double
@@ -93,6 +95,10 @@
p->uid == 0 || p->euid == 0)
points /= 4;

+ /* Much the same goes for processes with low UIDs */
+ if(p->uid < 100 || p->euid < 100)
+ points /= 2;
+
/*
* We don't want to kill a process with direct hardware access.
* Not only could that mess up the hardware, but usually users
@@ -192,12 +198,20 @@
int out_of_memory(void)
{
struct sysinfo swp_info;
+ long free;

/* Enough free memory? Not OOM. */
- if (nr_free_pages() > freepages.min)
+ free = nr_free_pages();
+ if (free > freepages.min)
+ return 0;
+
+ if (free + nr_inactive_clean_pages() > freepages.low)
return 0;

- if (nr_free_pages() + nr_inactive_clean_pages() > freepages.low)
+ /* Buffers and caches can be freed up (Jonathan "Chromatix" Morton) */
+ free += atomic_read(&buffermem_pages);
+ free += atomic_read(&page_cache_size);
+ if (free > freepages.low)
return 0;

/* Enough swap space left? Not OOM. */
Only in linux/mm/: oom_kill.c~
--- end oom-patch.diff ---
--------------------------------------------------------------
from: Jonathan "Chromatix" Morton
mail: chromi@cyberspace.org (not for attachments)
big-mail: chromatix@penguinpowered.com
uni-mail: j.d.morton@lancaster.ac.uk
The key to knowledge is not to rely on people to teach you it.

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