Re: [PATCH] Prevent OOM from killing init

[Jonathan Morton <chromi@cyberspace.org>]

>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.

Get VNC Server for Macintosh from http://www.chromatix.uklinux.net/vnc/

-----BEGIN GEEK CODE BLOCK-----
Version 3.12
GCS$/E/S dpu(!) s:- a20 C+++ UL++ P L+++ E W+ N- o? K? w--- O-- M++$ V? PS PE- Y+ PGP++ t- 5- X- R !tv b++ DI+++ D G e+ h+ r++ y+(*)
-----END GEEK CODE BLOCK-----