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

Commit Message

Alexander Potapenko Sept. 5, 2022, 12:24 p.m. UTC

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 <[e-mail]>
Acked-by: Marco Elver <[e-mail]>

---
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(+)

Patch

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;