Re: Suspicious error for CMA stress test

Joonsoo Kim
2016-03-07T04:34:17Z

On Fri, Mar 04, 2016 at 03:35:26PM +0800, Hanjun Guo wrote:
> On 2016/3/4 14:38, Joonsoo Kim wrote:
> > On Fri, Mar 04, 2016 at 02:05:09PM +0800, Hanjun Guo wrote:
> >> On 2016/3/4 12:32, Joonsoo Kim wrote:
> >>> On Fri, Mar 04, 2016 at 11:02:33AM +0900, Joonsoo Kim wrote:
> >>>> On Thu, Mar 03, 2016 at 08:49:01PM +0800, Hanjun Guo wrote:
> >>>>> On 2016/3/3 15:42, Joonsoo Kim wrote:
> >>>>>> 2016-03-03 10:25 GMT+09:00 Laura Abbott <labbott@redhat.com>:
> >>>>>>> (cc -mm and Joonsoo Kim)
> >>>>>>>
> >>>>>>>
> >>>>>>> On 03/02/2016 05:52 AM, Hanjun Guo wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> I came across a suspicious error for CMA stress test:
> >>>>>>>>
> >>>>>>>> Before the test, I got:
> >>>>>>>> -bash-4.3# cat /proc/meminfo | grep Cma
> >>>>>>>> CmaTotal: 204800 kB
> >>>>>>>> CmaFree: 195044 kB
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> After running the test:
> >>>>>>>> -bash-4.3# cat /proc/meminfo | grep Cma
> >>>>>>>> CmaTotal: 204800 kB
> >>>>>>>> CmaFree: 6602584 kB
> >>>>>>>>
> >>>>>>>> So the freed CMA memory is more than total..
> >>>>>>>>
> >>>>>>>> Also the the MemFree is more than mem total:
> >>>>>>>>
> >>>>>>>> -bash-4.3# cat /proc/meminfo
> >>>>>>>> MemTotal: 16342016 kB
> >>>>>>>> MemFree: 22367268 kB
> >>>>>>>> MemAvailable: 22370528 kB
> >>>>> [...]
> >>>>>>> I played with this a bit and can see the same problem. The sanity
> >>>>>>> check of CmaFree < CmaTotal generally triggers in
> >>>>>>> __move_zone_freepage_state in unset_migratetype_isolate.
> >>>>>>> This also seems to be present as far back as v4.0 which was the
> >>>>>>> first version to have the updated accounting from Joonsoo.
> >>>>>>> Were there known limitations with the new freepage accounting,
> >>>>>>> Joonsoo?
> >>>>>> I don't know. I also played with this and looks like there is
> >>>>>> accounting problem, however, for my case, number of free page is slightly less
> >>>>>> than total. I will take a look.
> >>>>>>
> >>>>>> Hanjun, could you tell me your malloc_size? I tested with 1 and it doesn't
> >>>>>> look like your case.
> >>>>> I tested with malloc_size with 2M, and it grows much bigger than 1M, also I
> >>>>> did some other test:
> >>>> Thanks! Now, I can re-generate erronous situation you mentioned.
> >>>>
> >>>>> - run with single thread with 100000 times, everything is fine.
> >>>>>
> >>>>> - I hack the cam_alloc() and free as below [1] to see if it's lock issue, with
> >>>>> the same test with 100 multi-thread, then I got:
> >>>> [1] would not be sufficient to close this race.
> >>>>
> >>>> Try following things [A]. And, for more accurate test, I changed code a bit more
> >>>> to prevent kernel page allocation from cma area [B]. This will prevent kernel
> >>>> page allocation from cma area completely so we can focus cma_alloc/release race.
> >>>>
> >>>> Although, this is not correct fix, it could help that we can guess
> >>>> where the problem is.
> >>> More correct fix is something like below.
> >>> Please test it.
> >> Hmm, this is not working:
> > Sad to hear that.
> >
> > Could you tell me your system's MAX_ORDER and pageblock_order?
> >
>
> MAX_ORDER is 11, pageblock_order is 9, thanks for your help!

Hmm... that's same with me.

Below is similar fix that prevents buddy merging when one of buddy's
migrate type, but, not both, is MIGRATE_ISOLATE. In fact, I have
no idea why previous fix (more correct fix) doesn't work for you.
(It works for me.) But, maybe there is a bug on the fix
so I make new one which is more general form. Please test it.

Thanks.

---------->8-------------
>From dd41e348572948d70b935fc24f82c096ff0fb417 Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Date: Fri, 4 Mar 2016 13:28:17 +0900
Subject: [PATCH] mm/cma: fix race

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
mm/page_alloc.c | 33 +++++++++++++++++++--------------
1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c6c38ed..d80d071 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -620,8 +620,8 @@ static inline void rmv_page_order(struct page *page)
*
* For recording page's order, we use page_private(page).
*/
-static inline int page_is_buddy(struct page *page, struct page *buddy,
- unsigned int order)
+static inline int page_is_buddy(struct zone *zone, struct page *page,
+ struct page *buddy, unsigned int order)
{
if (!pfn_valid_within(page_to_pfn(buddy)))
return 0;
@@ -644,6 +644,20 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
if (page_zone_id(page) != page_zone_id(buddy))
return 0;

+ if (IS_ENABLED(CONFIG_CMA) &&
+ unlikely(has_isolate_pageblock(zone)) &&
+ unlikely(order >= pageblock_order)) {
+ int page_mt, buddy_mt;
+
+ page_mt = get_pageblock_migratetype(page);
+ buddy_mt = get_pageblock_migratetype(buddy);
+
+ if (page_mt != buddy_mt &&
+ (is_migrate_isolate(page_mt) ||
+ is_migrate_isolate(buddy_mt)))
+ return 0;
+ }
+
VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);

return 1;
@@ -691,17 +705,8 @@ static inline void __free_one_page(struct page *page,
VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);

VM_BUG_ON(migratetype == -1);
- if (is_migrate_isolate(migratetype)) {
- /*
- * We restrict max order of merging to prevent merge
- * between freepages on isolate pageblock and normal
- * pageblock. Without this, pageblock isolation
- * could cause incorrect freepage accounting.
- */
- max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
- } else {
+ if (!is_migrate_isolate(migratetype))
__mod_zone_freepage_state(zone, 1 << order, migratetype);
- }

page_idx = pfn & ((1 << max_order) - 1);

@@ -711,7 +716,7 @@ static inline void __free_one_page(struct page *page,
while (order < max_order - 1) {
buddy_idx = __find_buddy_index(page_idx, order);
buddy = page + (buddy_idx - page_idx);
- if (!page_is_buddy(page, buddy, order))
+ if (!page_is_buddy(zone, page, buddy, order))
break;
/*
* Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
@@ -745,7 +750,7 @@ static inline void __free_one_page(struct page *page,
higher_page = page + (combined_idx - page_idx);
buddy_idx = __find_buddy_index(combined_idx, order + 1);
higher_buddy = higher_page + (buddy_idx - combined_idx);
- if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
+ if (page_is_buddy(zone, higher_page, higher_buddy, order + 1)) {
list_add_tail(&page->lru,
&zone->free_area[order].free_list[migratetype]);
goto out;
--
1.9.1