On 5/12/22 13:41, Minchan Kim wrote:
Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
so current is_pinnable_page could miss CMA pages which has MIGRATE_
ISOLATE. It ends up pinning CMA pages as longterm at pin_user_pages
APIs so CMA allocation keep failed until the pin is released.
CPU 0 CPU 1 - Task B
cma_alloc
alloc_contig_range
pin_user_pages_fast(FOLL_LONGTERM)
change pageblock as MIGRATE_ISOLATE
internal_get_user_pages_fast
lockless_pages_from_mm
gup_pte_range
try_grab_folio
is_pinnable_page
return true;
So, pinned the page successfully.
page migration failure with pinned page
..
.. After 30 sec
unpin_user_page(page)
CMA allocation succeeded after 30 sec.
The CMA allocation path protects the migration type change race
using zone->lock but what GUP path need to know is just whether the
page is on CMA area or not rather than exact migration type.
Thus, we don't need zone->lock but just checks migration type in
either of (MIGRATE_ISOLATE and MIGRATE_CMA).
Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
rejecting of pinning pages on MIGRATE_ISOLATE pageblocks even
though it's neither CMA nor movable zone if the page is temporarily
unmovable. However, such a migration failure by unexpected temporal
refcount holding is general issue, not only come from MIGRATE_ISOLATE
and the MIGRATE_ISOLATE is also transient state like other temporal
elevated refcount problem.
Cc: "Paul E . McKenney" <paulmck@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
* from v4 - https://lore.kernel.org/all/20220510211743.95831-1-minchan@xxxxxxxxxx/
* clarification why we need READ_ONCE - Paul
* Adding a comment about READ_ONCE - John
* from v3 - https://lore.kernel.org/all/20220509153430.4125710-1-minchan@xxxxxxxxxx/
* Fix typo and adding more description - akpm
* from v2 - https://lore.kernel.org/all/20220505064429.2818496-1-minchan@xxxxxxxxxx/
* Use __READ_ONCE instead of volatile - akpm
* from v1 - https://lore.kernel.org/all/20220502173558.2510641-1-minchan@xxxxxxxxxx/
* fix build warning - lkp
* fix refetching issue of migration type
* add side effect on !ZONE_MOVABLE and !MIGRATE_CMA in description - david
include/linux/mm.h | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6acca5cecbc5..2d7a5d87decd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1625,8 +1625,20 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
#ifdef CONFIG_MIGRATION
static inline bool is_pinnable_page(struct page *page)
{
- return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
- is_zero_pfn(page_to_pfn(page));
+#ifdef CONFIG_CMA
+ /*
+ * Defend against future compiler LTO features, or code refactoring
+ * that inlines the above function, by forcing a single read. Because,
+ * this routine races with set_pageblock_migratetype(), and we want to
+ * avoid reading zero, when actually one or the other flags was set.
+ */
The most interesting line got dropped in this version. :)
This is missing:
int __mt = get_pageblock_migratetype(page);
Assuming that that is restored, please feel free to add:
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
thanks,
--
John Hubbard
NVIDIA
+ int mt = __READ_ONCE(__mt);
+
+ if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
+ return false;
+#endif
+
+ return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
}
#else
static inline bool is_pinnable_page(struct page *page)
