Re: Suspicious error for CMA stress test

Joonsoo Kim
2016-03-04T04:32:09Z

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.

It checks problematic buddy merging and prevent it.
I will try to find another way that is less intrusive for freepath performance.

Thanks.

---------------->8-----------------------
>From 855cb11368487a0f02a5ad5b3d9de375dfbb061c 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 | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c6c38ed..a01c3b5 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 +644,12 @@ 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) &&
+ has_isolate_pageblock(zone) &&
+ order >= pageblock_order &&
+ is_migrate_isolate(get_pageblock_migratetype(buddy)))
+ return 0;
+
VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);

return 1;
@@ -711,7 +717,7 @@ 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 +751,7 @@ 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