[PATCH] mm/kmemleak: Move up cond_resched() call in page scanning loop

[PATCH] mm/kmemleak: Move up cond_resched() call in page scanning loop

[Waiman Long]

Commit bde5f6bc68db ("kmemleak: add scheduling point to kmemleak_scan()")
added a cond_resched() call to the struct page scanning loop to prevent
soft lockup from happening. However, soft lockup can still happen in
that loop in some corner cases when the pages that satisfy the "!(pfn &
63)" check are skipped for some reasons.

Fix this corner case by moving up the cond_resched() check so that it
will be called every 64 pages unconditionally.

Fixes: bde5f6bc68db ("kmemleak: add scheduling point to kmemleak_scan()")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/kmemleak.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index a2d34226e3c8..114d8dcdcf70 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1579,6 +1579,9 @@ static void kmemleak_scan(void)
 		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
 			struct page *page = pfn_to_online_page(pfn);
 
+			if (!(pfn & 63))
+				cond_resched();
+
 			if (!page)
 				continue;
 
@@ -1589,8 +1592,6 @@ static void kmemleak_scan(void)
 			if (page_count(page) == 0)
 				continue;
 			scan_block(page, page + 1, NULL);
-			if (!(pfn & 63))
-				cond_resched();
 		}
 	}
 	put_online_mems();
-- 
2.31.1

Re: [PATCH] mm/kmemleak: Move up cond_resched() call in page scanning loop

[Catalin Marinas]

On Fri, Aug 25, 2023 at 12:49:47PM -0400, Waiman Long wrote:
> Commit bde5f6bc68db ("kmemleak: add scheduling point to kmemleak_scan()")
> added a cond_resched() call to the struct page scanning loop to prevent
> soft lockup from happening. However, soft lockup can still happen in
> that loop in some corner cases when the pages that satisfy the "!(pfn &
> 63)" check are skipped for some reasons.
> 
> Fix this corner case by moving up the cond_resched() check so that it
> will be called every 64 pages unconditionally.
> 
> Fixes: bde5f6bc68db ("kmemleak: add scheduling point to kmemleak_scan()")
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>