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

[Minchan Kim]

On Sat, Mar 05, 2011 at 07:41:26PM +0300, Andrew Vagin wrote:
> On 03/05/2011 06:53 PM, Minchan Kim wrote:
> >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@xxxxxxxxxx>
> >>>>---
> >>>>  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.
> balance_pgdat set zone->all_unreclaimable, but the problem is that
> it is cleaned late.

Yes. It can be delayed by pcp so (zone->all_unreclaimable = true) is
a false alram since zone have a free page and it can be returned 
to free list by drain_all_pages in next turn.

> 
> The problem is that zone->all_unreclaimable = True, but
> zone_reclaimable() returns True too.

Why is it a problem? 
If zone->all_unreclaimable gives a false alram, we does need to check
it again by zone_reclaimable call.

If we believe a false alarm and give up the reclaim, maybe we have to make
unnecessary oom kill.

> 
> zone->all_unreclaimable will be cleaned in free_*_pages, but this
> may be late. It is enough allocate one page from page cache, that
> zone_reclaimable() returns True and zone->all_unreclaimable becomes
> True.
> >>>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?
> Yes.

Hmm.. Although it solves the problem, I think it's not a good idea that
depends on false alram and give up the retry.


> >>>>-- 
> >>>>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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