Re: [PATCH] mm: check zone->all_unreclaimable in all_unreclaimable()

[Minchan Kim <minchan.kim@gmail.com>]

On Sat, Mar 05, 2011 at 06:34:37PM +0300, Andrew Vagin wrote:
> On 03/05/2011 06:20 PM, Minchan Kim wrote:
> >On Sat, Mar 05, 2011 at 02:44:16PM +0300, Andrey Vagin wrote:
> >>Check zone->all_unreclaimable in all_unreclaimable(), otherwise the
> >>kernel may hang up, because shrink_zones() will do nothing, but
> >>all_unreclaimable() will say, that zone has reclaimable pages.
> >>
> >>do_try_to_free_pages()
> >>	shrink_zones()
> >>		 for_each_zone
> >>			if (zone->all_unreclaimable)
> >>				continue
> >>	if !all_unreclaimable(zonelist, sc)
> >>		return 1
> >>
> >>__alloc_pages_slowpath()
> >>retry:
> >>	did_some_progress = do_try_to_free_pages(page)
> >>	...
> >>	if (!page&&  did_some_progress)
> >>		retry;
> >>
> >>Signed-off-by: Andrey Vagin<avagin@openvz.org>
> >>---
> >>  mm/vmscan.c |    2 ++
> >>  1 files changed, 2 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>index 6771ea7..1c056f7 100644
> >>--- a/mm/vmscan.c
> >>+++ b/mm/vmscan.c
> >>@@ -2002,6 +2002,8 @@ static bool all_unreclaimable(struct zonelist *zonelist,
> >>
> >>  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
> >>  			gfp_zone(sc->gfp_mask), sc->nodemask) {
> >>+		if (zone->all_unreclaimable)
> >>+			continue;
> >>  		if (!populated_zone(zone))
> >>  			continue;
> >>  		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
> >
> >zone_reclaimable checks it. Isn't it enough?
> I sent one more patch [PATCH] mm: skip zombie in OOM-killer.
> This two patches are enough.

Sorry if I confused you. 
I mean zone->all_unreclaimable become true if !zone_reclaimable in balance_pgdat.
zone_reclaimable compares recent pages_scanned with the number of zone lru pages.
So too many page scanning in small lru pages makes the zone to unreclaimable zone.

In all_unreclaimable, we calls zone_reclaimable to detect it.
It's the same thing with your patch.

> >Does the hang up really happen or see it by code review?
> Yes. You can reproduce it for help the attached python program. It's
> not very clever:)
> It make the following actions in loop:
> 1. fork
> 2. mmap
> 3. touch memory
> 4. read memory
> 5. munmmap

It seems the test program makes fork bombs and memory hogging.
If you applied this patch, the problem is gone?

> 
> >>-- 
> >>1.7.1
> >>
> >>--
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> 

> import sys, time, mmap, os
> from subprocess import Popen, PIPE
> import random
> 
> global mem_size
> 
> def info(msg):
> 	pid = os.getpid()
> 	print >> sys.stderr, "%s: %s" % (pid, msg)
> 	sys.stderr.flush()
> 
> 
> 
> def memory_loop(cmd = "a"):
> 	"""
> 	cmd may be:
> 		c: check memory
> 		else: touch memory
> 	"""
> 	c = 0
> 	for j in xrange(0, mem_size):
> 		if cmd == "c":
> 			if f[j<<12] != chr(j % 255):
> 				info("Data corruption")
> 				sys.exit(1)
> 		else:
> 			f[j<<12] = chr(j % 255)
> 
> while True:
> 	pid = os.fork()
> 	if (pid != 0):
> 		mem_size = random.randint(0, 56 * 4096)
> 		f = mmap.mmap(-1, mem_size << 12, mmap.MAP_ANONYMOUS|mmap.MAP_PRIVATE)
> 		memory_loop()
> 		memory_loop("c")
> 		f.close()


-- 
Kind regards,
Minchan Kim

--
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