[RFC PATCH v3 0/4] fix freepage count problems in memory isolation

From: Joonsoo Kim
Date: Tue Aug 26 2014 - 04:08:19 EST

This is version 3 patchset which is improved and minimized version of
version 1 to fix freepage accounting problem during memory isolation.
I tried different approach in version 2, but, it looks really complicated
so I change my mind to improve version 1. You can see version 1, 2 in
following links [1] [2], respectively.

IMO, this v3 is better than v2, because this is simpler than v2 so
better for maintainance and this doesn't change pageblock isolation
logic so it is much easier to backport.

This problems are found by testing my patchset [3]. There are some race
conditions on pageblock isolation and these race cause incorrect
freepage count.

Before describing bugs itself, I first explain definition of freepage.

1. pages on buddy list are counted as freepage.
2. pages on isolate migratetype buddy list are *not* counted as freepage.
3. pages on cma buddy list are counted as CMA freepage, too.

Now, I describe problems and related patch.

Patch 1: There is race conditions on getting pageblock migratetype that
it results in misplacement of freepages on buddy list, incorrect
freepage count and un-availability of freepage.

Patch 2: Freepages on pcp list could have stale cached information to
determine migratetype of buddy list to go. This causes misplacement
of freepages on buddy list and incorrect freepage count.

Patch 4: Merging between freepages on different migratetype of
pageblocks will cause freepages accouting problem. This patch fixes it.

Without patchset [3], above problem doesn't happens on my CMA allocation
test, because CMA reserved pages aren't used at all. So there is no
chance for above race.

With patchset [3], I did simple CMA allocation test and get below result.

- Virtual machine, 4 cpus, 1024 MB memory, 256 MB CMA reservation
- run kernel build (make -j16) on background
- 30 times CMA allocation(8MB * 30 = 240MB) attempts in 5 sec interval
- Result: more than 5000 freepage count are missed

With patchset [3] and this patchset, I found that no freepage count are
missed so that I conclude that problems are solved.

These problems can be possible on memory hot remove users, although
I didn't check it further.

This patchset is based on linux-next-20140826.
Please see individual patches for more information.


[1]: https://lkml.org/lkml/2014/7/4/79
[2]: lkml.org/lkml/2014/8/6/52
[3]: Aggressively allocate the pages on cma reserved memory

Joonsoo Kim (4):
mm/page_alloc: fix incorrect isolation behavior by rechecking
mm/page_alloc: add freepage on isolate pageblock to correct buddy
mm/page_alloc: move migratetype recheck logic to __free_one_page()
mm/page_alloc: restrict max order of merging on isolated pageblock

include/linux/mmzone.h | 4 ++++
include/linux/page-isolation.h | 8 ++++++++
mm/page_alloc.c | 28 +++++++++++++++++++---------
mm/page_isolation.c | 2 ++
4 files changed, 33 insertions(+), 9 deletions(-)


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/