mm: reset migratetype if the range spans two pageblocks
Subsystems (2)
PATCH 1/1 mm: reset migratetype if the range spans two pageblocks
__rmqueue_fallback() will change the migratetype of pageblock, so it is possible that two continuous pageblocks have different migratetypes.
When freeing all pages of the two blocks, they will be merged to 4M, and added to the buddy list which the migratetype is the first pageblock's.
If later alloc some pages and split the 4M, the second pageblock will be added to the buddy list, and the migratetype is the first pageblock's, so it is different from the its pageblock's.
That means the page in buddy list's migratetype is different from the page in pageblock's migratetype. This will make confusion.
However,if we change the hotpath, it will be performance degradation, so any better ideas?
--- mm/page_alloc.c | 3 +++ 1 file changed, 3 insertions(+)
mm/page_alloc.c +3 -0
@@ -726,6 +726,9 @@ static inline void __free_one_page(struct page *page,list_add (&page->lru, &zone->free_area[order].free_list[migratetype]); out: zone->free_area[order].nr_free++; +/* If the range spans two pageblocks, reset the migratetype. */ +if (order > pageblock_order) +change_pageblock_range (page, order, migratetype); }static inline int free_pages_check (struct page *page)
Signature · 1 line
2.0.0
Quoted 12 lines · Xishi Qiu
__rmqueue_fallback()will change the migratetype of pageblock, so it is possible that two continuous pageblocks have different migratetypes.When freeing all pages of the two blocks, they will be merged to 4M, and added to the buddy list which the migratetype is the first pageblock's.
If later alloc some pages and split the 4M, the second pageblock will be added to the buddy list, and the migratetype is the first pageblock's, so it is different from the its pageblock's.
Quoted 6 lines
That means the page in buddy list's migratetype is different from the page in pageblock's migratetype. This will make confusion.
However,if we change the hotpath, it will be performance degradation, so any better ideas?
How about using get_pfnblock_migratetype() to get the pageblock's migratetype first, and compare them?
Thanks,
Xishi Qiu
Quoted 19 lines
Signed-off-by Xishi Qiu <[email protected]>--- mm/page_alloc.c | 3 +++ 1 file changed, 3 insertions(+)mm/page_alloc.c +3
-0@@ -726,6 +726,9 @@ static inline void __free_one_page(struct page *page,list_add (&page->lru, &zone->free_area[order].free_list[migratetype]); out: zone->free_area[order].nr_free++; +/* If the range spans two pageblocks, reset the migratetype. */ +if (order > pageblock_order) +change_pageblock_range (page, order, migratetype); }static inline int free_pages_check (struct page *page)
kbuild test robot
build botreproduce:
Hi Xishi,
[auto build test ERROR on v4.3-rc5 -- if it's inappropriate base, please suggest rules for selecting the more suitable base]
url: https://github.com/0day-ci/linux/commits/Xishi-Qiu/mm-reset-migratetype-if-the-range-spans-two-pageblocks/20151016-180949 config: i386-randconfig-s1-201541 (attached as .config) reproduce: # save the attached .config to linux build tree make ARCH=i386
All error/warnings (new ones prefixed by >>):
mm/page_alloc.c: In function '__free_one_page':
>> mm/page_alloc.c:731:3: error: implicit declaration of function 'change_pageblock_range' [-Werror=implicit-function-declaration]
change_pageblock_range(page, order, migratetype);
^
mm/page_alloc.c: At top level:
>> mm/page_alloc.c:1515:13: warning: conflicting types for 'change_pageblock_range'
static void change_pageblock_range(struct page *pageblock_page,
^
>> mm/page_alloc.c:1515:13: error: static declaration of 'change_pageblock_range' follows non-static declaration
mm/page_alloc.c:731:3: note: previous implicit declaration of 'change_pageblock_range' was here
change_pageblock_range(page, order, migratetype);
^cc1: some warnings being treated as errors
vim +/change_pageblock_range +731 mm/page_alloc.c
725
726 list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
727 out:
728 zone->free_area[order].nr_free++;
729 /* If the range spans two pageblocks, reset the migratetype. */
730 if (order > pageblock_order)
> 731 change_pageblock_range(page, order, migratetype);
732 }
733
734 static inline int free_pages_check(struct page *page)
---
0-DAY kernel test infrastructure Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all Intel Corporation
Quoted 8 lines · Xishi Qiu
__rmqueue_fallback()will change the migratetype of pageblock, so it is possible that two continuous pageblocks have different migratetypes.When freeing all pages of the two blocks, they will be merged to 4M, and added to the buddy list which the migratetype is the first pageblock's.
Quoted 6 lines
If later alloc some pages and split the 4M, the second pageblock will be added to the buddy list, and the migratetype is the first pageblock's, so it is different from the its pageblock's.
That means the page in buddy list's migratetype is different from the page in pageblock's migratetype. This will make confusion.
So what will be the bad effects of this confusion? There are many situations where a free page (of any size) will be on different list than the pageblock's migratetype.
In case of full free pageblock, it IIRC doesn't really matter on which freelist it is, as the fallback scenarios for all migratetypes are trivial and non-fragmenting (just grab the whole pageblock, which means the migratetype is updated).
Maybe compaction will get it wrong when deciding which pageblocks are suitable for scanning, but that's not critical.
Quoted 2 lines
However,if we change the hotpath, it will be performance degradation, so any better ideas?
I don't see immediately a way to fix that outside of hotpath, and don't see the reasons being strong enough to fix it in the hotpath.
Quoted 12 lines
Signed-off-by Xishi Qiu <[email protected]>--- mm/page_alloc.c | 3 +++ 1 file changed, 3 insertions(+)mm/page_alloc.c +0
-0
mm/page_alloc.c static inline void __free_one_page(struct page *page, · line 734
Quoted 9 lines
mm/page_alloc.c +3
-0@@ -726,6 +726,9 @@ static inline void __free_one_page(struct page *page,list_add (&page->lru, &zone->free_area[order].free_list[migratetype]); out: zone->free_area[order].nr_free++; +/* If the range spans two pageblocks, reset the migratetype. */ +if (order > pageblock_order) +change_pageblock_range (page, order, migratetype); }static inline int free_pages_check (struct page *page)
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