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Subjectmore testing on 2.4.0-t9p[456] VM deadlocks

Hi,

want to summarize my observations wrt the VM-deadlock issue. Everything
tested on UP box bootet with mem=8M and 500M swap.

2.4.0-t9p4 (vanilla)
deadlocks almost everywhere (even in initscripts!), simple dd with
large enough bs deadlock's as soon as page_out should start - i.e. no
swapspace ever used. Fortunately, deadlock can be cured by SysRq+e.

2.4.0-t9p5+vmfixes-2.4.0-test9-A1
much better, but still some deadlock. Much harder to trigger in a
reproducible way. "make dep" on /usr/src/linux seems to be a good
candidate, initscripts too. At some time some (undetermined) process
hangs - this process remains the only one ever being current forever!
However EIP still changes according to SysRq+p. This behaviour can
not be solved by SysRq+[ekil]. SysRq+[sb] not succesful either,
so its time for unclean reboot und fsck (which deadlocks too).

2.4.0-t9p6 (vanilla)
very similar to last one. Put some printk() into it to find out, it
ends up looping forever due to goto try_again in __alloc_pages().
Just to give it a try I've added a counter to it restricting the
number of retries (to some arbitrary number - 5 in may case).
If this is reached I continue to the code following the goto.
result: no "deadlocks" anymore! I was even able to run the
initscripts and make bzImage. However, from time to time some
processes were killed due to lack of memory. sysctl/overcommit_memory
was 0 and there was plenty of free swapspace.
Finally I followed Ingo' suggestion to change __GFP_IO to __GFP_WAIT
in refill_inactive(). With this there was no process killed anymore!
I was able to complete make bzImage without anything going wrong.
Furthermore it was possible to start X and login to KDE - although
the system performance was far from being useable there was no
problem (besides disk activity). The box run at av. load 2 for
several hours doing almost nothing but page_in/out. My try_again
escape was hit quit often (several 1000 times) but the fallback to
the critical case seems to work. As I printk()'ed the value of
memory_pressure in this case I can tell the box was running at
memory_pressure of 30000-40000 for more than 5 hours with peaks
beyond 50000 when starting X/KDE e.g. - just in case these numbers
tell you something.

Taken together, the described fix seems to do the job on my system. The
patch vs. vanilla 2.4.0-t9p6 is included below. I would not claim it
to be "Right" or even more than a hack to find out what's going wrong.
Just want to give you some feedback.

PS: vmfixes-2.4.0-test9-B2 not yet tested - will do later.

Regards
Martin


diff -Nur linux-2.4.0-t9p6.orig/mm/page_alloc.c linux-2.4.0-t9p6/mm/page_alloc.c
--- linux-2.4.0-t9p6.orig/mm/page_alloc.c Mon Sep 25 01:08:52 2000
+++ linux-2.4.0-t9p6/mm/page_alloc.c Mon Sep 25 01:16:50 2000
@@ -295,6 +295,7 @@
int direct_reclaim = 0;
unsigned int gfp_mask = zonelist->gfp_mask;
struct page * page = NULL;
+ int retry = 5;

/*
* Allocations put pressure on the VM subsystem.
@@ -444,9 +445,14 @@
* processes, etc).
*/
if (gfp_mask & __GFP_WAIT) {
- try_to_free_pages(gfp_mask);
- memory_pressure++;
- goto try_again;
+ if (--retry > 0) {
+ try_to_free_pages(gfp_mask);
+ memory_pressure++;
+ goto try_again;
+ }
+ printk(KERN_CRIT "critical memory fallback\n");
+ printk("%s - memory_pressure = %i\n",
+ __FUNCTION__, memory_pressure);
}
}

diff -Nur linux-2.4.0-t9p6.orig/mm/vmscan.c linux-2.4.0-t9p6/mm/vmscan.c
--- linux-2.4.0-t9p6.orig/mm/vmscan.c Mon Sep 25 01:08:40 2000
+++ linux-2.4.0-t9p6/mm/vmscan.c Sun Sep 24 13:14:14 2000
@@ -891,7 +891,7 @@
do {
made_progress = 0;

- if (current->need_resched && (gfp_mask & __GFP_IO)) {
+ if (current->need_resched && (gfp_mask & __GFP_WAIT)) {
__set_current_state(TASK_RUNNING);
schedule();
}
-
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