[PATCH v6 19/44] kmsan: unpoison @tlb in arch_tlb_gather_mmu()

Alexander Potapenko
2022-09-05T12:28:58Z

This is an optimization to reduce stackdepot pressure.

struct mmu_gather contains 7 1-bit fields packed into a 32-bit unsigned
int value. The remaining 25 bits remain uninitialized and are never used,
but KMSAN updates the origin for them in zap_pXX_range() in mm/memory.c,
thus creating very long origin chains. This is technically correct, but
consumes too much memory.

Unpoisoning the whole structure will prevent creating such chains.

Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Marco Elver <elver@google.com>

---
v5:
-- updated description as suggested by Marco Elver

Link: https://linux-review.googlesource.com/id/I76abee411b8323acfdbc29bc3a60dca8cff2de77
---
mm/mmu_gather.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index a71924bd38c0d..add4244e5790d 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -1,6 +1,7 @@
#include <linux/gfp.h>
#include <linux/highmem.h>
#include <linux/kernel.h>
+#include <linux/kmsan-checks.h>
#include <linux/mmdebug.h>
#include <linux/mm_types.h>
#include <linux/mm_inline.h>
@@ -265,6 +266,15 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
bool fullmm)
{
+ /*
+ * struct mmu_gather contains 7 1-bit fields packed into a 32-bit
+ * unsigned int value. The remaining 25 bits remain uninitialized
+ * and are never used, but KMSAN updates the origin for them in
+ * zap_pXX_range() in mm/memory.c, thus creating very long origin
+ * chains. This is technically correct, but consumes too much memory.
+ * Unpoisoning the whole structure will prevent creating such chains.
+ */
+ kmsan_unpoison_memory(tlb, sizeof(*tlb));
tlb->mm = mm;
tlb->fullmm = fullmm;

--
2.37.2.789.g6183377224-goog