Since commit 1378a5ee451a ("mm: store compound_nr as well as
compound_order") the page[1].compound_nr must be explicitly set to 0 if
calling set_compound_order(page, 0).
This can lead to bugs if the caller of set_compound_order(page, 0) forgets
to explicitly set compound_nr=0. An example of this is commit ba9c1201beaa
("mm/hugetlb: clear compound_nr before freeing gigantic pages")
Collapse these calls into the set_compound_order by utilizing branchless
bitmaths [1].
[1] https://graphics.stanford.edu/~seander/bithacks.html#ConditionalSetOrClearBitsWithoutBranching
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
Author: Nico Pache <npache@xxxxxxxxxx>
---
include/linux/mm.h | 6 +++++-
mm/hugetlb.c | 6 ------
2 files changed, 5 insertions(+), 7 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6a05a3bc0a28..75b57c71781f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -938,9 +938,13 @@ static inline int head_compound_pincount(struct page *head)
static inline void set_compound_order(struct page *page, unsigned int order)
{
+ unsigned long shift = (1U << order);
page[1].compound_order = order;
#ifdef CONFIG_64BIT
- page[1].compound_nr = 1U << order;
+ // Branchless conditional:
+ // // order > 0 --> compound_nr = shift
+ // // order == 0 --> compound_nr = 0
+ page[1].compound_nr = shift ^ (-order ^ shift) & shift;
#endif
}
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3d9f4abec17c..706dec43a6a2 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1344,9 +1344,6 @@ static void __destroy_compound_gigantic_page(struct page *page,
}
set_compound_order(page, 0);
-#ifdef CONFIG_64BIT
- page[1].compound_nr = 0;
-#endif
__ClearPageHead(page);
}
@@ -1865,9 +1862,6 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
__ClearPageReserved(p);
}
set_compound_order(page, 0);
-#ifdef CONFIG_64BIT
- page[1].compound_nr = 0;
-#endif
__ClearPageHead(page);
return false;
}
--
2.38.1
