Date:	Tue, 4 Mar 2014 09:55:13 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/6] mm: add get_pageblock_migratetype_nolock() for cases
 where locking is undesirable

On Mon, Mar 03, 2014 at 02:54:09PM +0100, Vlastimil Babka wrote:
> On 03/03/2014 09:22 AM, Joonsoo Kim wrote:
> >On Fri, Feb 28, 2014 at 03:15:00PM +0100, Vlastimil Babka wrote:
> >>In order to prevent race with set_pageblock_migratetype, most of calls to
> >>get_pageblock_migratetype have been moved under zone->lock. For the remaining
> >>call sites, the extra locking is undesirable, notably in free_hot_cold_page().
> >>
> >>This patch introduces a _nolock version to be used on these call sites, where
> >>a wrong value does not affect correctness. The function makes sure that the
> >>value does not exceed valid migratetype numbers. Such too-high values are
> >>assumed to be a result of race and caller-supplied fallback value is returned
> >>instead.
> >>
> >>Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> >>---
> >>  include/linux/mmzone.h | 24 ++++++++++++++++++++++++
> >>  mm/compaction.c        | 14 +++++++++++---
> >>  mm/memory-failure.c    |  3 ++-
> >>  mm/page_alloc.c        | 22 +++++++++++++++++-----
> >>  mm/vmstat.c            |  2 +-
> >>  5 files changed, 55 insertions(+), 10 deletions(-)
> >>
> >>diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> >>index fac5509..7c3f678 100644
> >>--- a/include/linux/mmzone.h
> >>+++ b/include/linux/mmzone.h
> >>@@ -75,6 +75,30 @@ enum {
> >>
> >>  extern int page_group_by_mobility_disabled;
> >>
> >>+/*
> >>+ * When called without zone->lock held, a race with set_pageblock_migratetype
> >>+ * may result in bogus values. Use this variant only when this does not affect
> >>+ * correctness, and taking zone->lock would be costly. Values >= MIGRATE_TYPES
> >>+ * are considered to be a result of this race and the value of race_fallback
> >>+ * argument is returned instead.
> >>+ */
> >>+static inline int get_pageblock_migratetype_nolock(struct page *page,
> >>+	int race_fallback)
> >>+{
> >>+	int ret = get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
> >>+
> >>+	if (unlikely(ret >= MIGRATE_TYPES))
> >>+		ret = race_fallback;
> >>+
> >>+	return ret;
> >>+}
> >
> >Hello, Vlastimil.
> >
> >First of all, thanks for nice work!
> >I have another opinion about this implementation. It can be wrong, so if it
> >is wrong, please let me know.
> 
> Thanks, all opinions/reviewing is welcome :)
> 
> >Although this implementation would close the race which triggers NULL dereference,
> >I think that this isn't enough if you have a plan to add more
> >{start,undo}_isolate_page_range().
> >
> >Consider that there are lots of {start,undo}_isolate_page_range() calls
> >on the system without CMA.
> >
> >bit representation of migratetype is like as following.
> >
> >MIGRATE_MOVABLE = 010
> >MIGRATE_ISOLATE = 100
> >
> >We could read following values as migratetype of the page on movable pageblock
> >if race occurs.
> >
> >start_isolate_page_range() case: 010 -> 100
> >010, 000, 100
> >
> >undo_isolate_page_range() case: 100 -> 010
> >100, 110, 010
> >
> >Above implementation prevents us from getting 110, but, it can't prevent us from
> >getting 000, that is, MIGRATE_UNMOVABLE. If this race occurs in free_hot_cold_page(),
> >this page would go into unmovable pcp and then allocated for that migratetype.
> >It results in more fragmented memory.
> 
> Yes, that can happen. But I would expect it to be negligible to
> other causes of fragmentation. But I'm not at this moment sure how
> often {start,undo}_isolate_page_range() would be called in the end.
> Certainly
> not as often as in the development patch which is just to see if
> that can improve anything. Because it will have its own overhead
> (mostly for zone->lock) that might be too large. But good point, I
> will try to quantify this.
> 
> >
> >Consider another case that system enables CONFIG_CMA,
> >
> >MIGRATE_MOVABLE = 010
> >MIGRATE_ISOLATE = 101
> >
> >start_isolate_page_range() case: 010 -> 101
> >010, 011, 001, 101
> >
> >undo_isolate_page_range() case: 101 -> 010
> >101, 100, 110, 010
> >
> >This can results in totally different values and this also makes the problem
> >mentioned above. And, although this doesn't cause any problem on CMA for now,
> >if another migratetype is introduced or some migratetype is removed, it can cause
> >CMA typed page to go into other migratetype and makes CMA permanently failed.
> 
> This should actually be no problem for free_hot_cold_page() as any
> migratetype >= MIGRATE_PCPTYPES will defer to free_one_page() which
> will reread migratetype under zone->lock. So as long as
> MIGRATE_PCPTYPES does not include a migratetype with such dangerous
> "permanently failed" properties, it should be good. And I doubt such
> migratetype would be added to pcptypes. But of course, anyone adding
> new migratetype would have to reconsider each
> get_pageblock_migratetype_nolock() call for such potential problems.

Please let me explain more.
Now CMA page can have following race values.

MIGRATE_CMA = 100
MIGRATE_ISOLATE = 101

start_isolate_page_range(): 100 -> 101
100, 101
undo_isolate_page_range(): 101 -> 100
101, 100

So, race doesn't cause any big problem.

But, as you mentioned in earlier patch, it could get worse if MIGRATE_RESERVE
is removed. It doesn't happen until now, but, it can be possible.

In that case,

MIGRATE_CMA = 011
MIGRATE_ISOLATE = 100

start_isolate_page_range(): 011 -> 100
011, 010, 000, 100
undo_isolate_page_range(): 100 -> 011
100, 101, 111, 011

If this race happens, CMA page can go into MIGRATE_UNMOVABLE list, because
"migratetpye >= MIGRATE_PCPTYPES" can't prevent it, and this could make
CMA permanently failed.

I think that to dump the responsibility on developer who want to add/remove migratetype
is not reasonable and doesn't work well, because they may not have enough background
knowledge. I hope to close the possible race more in this time.

Thanks.

> 
> >To close this kind of races without dependency how many pageblock isolation occurs,
> >I recommend that you use separate pageblock bits for MIGRATE_CMA, MIGRATE_ISOLATE
> >and use accessor function whenver we need to check migratetype. IMHO, it may not
> >impose much overhead.
> 
> That could work in case the fragmentation is confirmed to be a problem.
> 
> Thanks,
> Vlastimil
> 
> >How about it?
> >
> >Thanks.
> >
> >--
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> >
> 
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