[RFC,v4,29/42] kmsan: unpoison |tlb| in arch_tlb_gather_mmu()

Commit Message

Alexander Potapenko Dec. 20, 2019, 6:49 p.m. UTC

This is a hack 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]>
To: Alexander Potapenko <[e-mail]>
Cc: Vegard Nossum <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Marco Elver <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: [e-mail]

---

v4:
 - removed a TODO, updated patch description

Change-Id: I22a201e7e4f67ed74f8129072f12e5351b26103a
---
 mm/mmu_gather.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Patch

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 7d70e5c78f97..28105c267268 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/pagemap.h>
@@ -206,6 +207,15 @@  void tlb_flush_mmu(struct mmu_gather *tlb)
 void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
 			unsigned long start, unsigned long end)
 {
+	/*
+	 * 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_shadow(tlb, sizeof(*tlb));
 	tlb->mm = mm;
 
 	/* Is it from 0 to ~0? */