On Wed, Mar 25, 2020 at 5:14 PM <glider@xxxxxxxxxx> wrote:
>
> 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 <glider@xxxxxxxxxx>
> To: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> ---
>
> v4:
> - removed a TODO, updated patch description
>
> Change-Id: I22a201e7e4f67ed74f8129072f12e5351b26103a
> ---
> mm/mmu_gather.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index a3538cb2bcbee..d3d57c276e301 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>
> @@ -264,6 +265,15 @@ 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? */
> --
> 2.25.1.696.g5e7596f4ac-goog
>
