linux-kernel - [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

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Message-ID: <20251205175037.1287366-1-lorenzo.stoakes@oracle.com>
Date: Fri,  5 Dec 2025 17:50:37 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        oliver.sang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

Commit 2b6a3f061f11 ("mm: declare VMA flags by bit") significantly changed
how VMA flags are declared, utilising an enum of VMA bit values and
ifdef-fery VM_xxx flag declarations via macro.

As part of this change, it uses INIT_VM_FLAG() to define VM_xxx flags from
the newly introduced VMA bit numbers.

However, use of this macro results in apparently unfortunate macro
expansion and resulted in a performance degradation.This appears to be due
to the (__force int), which is required for the sparse typechecking to
work.

Avoid macro expansion issues by simply using 1UL << bitnum.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202512041634.150c7e4f-lkp@intel.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---

Andrew - note I've referenced the linux-next commit number above, could you
replace with the upstream commit hash once your PR is taken? Thanks!

 include/linux/mm.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a2f38fb68840..c4438b30c140 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -395,7 +395,8 @@ enum {
 #undef DECLARE_VMA_BIT
 #undef DECLARE_VMA_BIT_ALIAS

-#define INIT_VM_FLAG(name) BIT((__force int) VMA_ ## name ## _BIT)
+#define INIT_VM_FLAG(name) (1UL << (__force int)(VMA_ ## name ## _BIT))
+
 #define VM_READ		INIT_VM_FLAG(READ)
 #define VM_WRITE	INIT_VM_FLAG(WRITE)
 #define VM_EXEC		INIT_VM_FLAG(EXEC)
--
2.52.0