Re: [PATCH v4 19/45] kmsan: unpoison @tlb in arch_tlb_gather_mmu()

[Marco Elver]

On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> This is a hack to reduce stackdepot pressure.

Will it cause false negatives or other issues? If not, I'd just call
it an optimization and not a hack.

> 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>

Acked-by: Marco Elver <elver@xxxxxxxxxx>

> ---
> 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;
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>