Re: [PATCH mm v3 28/38] kasan, page_alloc: allow skipping memory init for HW_TAGS

[Kuan-Ying Lee]

On Tue, 2021-12-14 at 05:54 +0800, andrey.konovalov@xxxxxxxxx wrote:
> From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> Add a new GFP flag __GFP_SKIP_ZERO that allows to skip memory
> initialization. The flag is only effective with HW_TAGS KASAN.
> 
> This flag will be used by vmalloc code for page_alloc allocations
> backing vmalloc() mappings in a following patch. The reason to skip
> memory initialization for these pages in page_alloc is because
> vmalloc
> code will be initializing them instead.
> 
> With the current implementation, when __GFP_SKIP_ZERO is provided,
> __GFP_ZEROTAGS is ignored. This doesn't matter, as these two flags
> are
> never provided at the same time. However, if this is changed in the
> future, this particular implementation detail can be changed as well.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> ---
> 
> Changes v2->v3:
> - Update patch description.
> 
> Changes v1->v2:
> - Add this patch.
> ---
>  include/linux/gfp.h | 16 +++++++++++-----
>  mm/page_alloc.c     | 13 ++++++++++++-
>  2 files changed, 23 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 6781f84345d1..b8b1a7198186 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -54,10 +54,11 @@ struct vm_area_struct;
>  #define ___GFP_THISNODE		0x200000u
>  #define ___GFP_ACCOUNT		0x400000u
>  #define ___GFP_ZEROTAGS		0x800000u
> -#define ___GFP_SKIP_KASAN_UNPOISON	0x1000000u
> -#define ___GFP_SKIP_KASAN_POISON	0x2000000u
> +#define ___GFP_SKIP_ZERO	0x1000000u
> +#define ___GFP_SKIP_KASAN_UNPOISON	0x2000000u
> +#define ___GFP_SKIP_KASAN_POISON	0x4000000u
>  #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP	0x4000000u
> +#define ___GFP_NOLOCKDEP	0x8000000u
>  #else
>  #define ___GFP_NOLOCKDEP	0
>  #endif
> @@ -230,7 +231,11 @@ struct vm_area_struct;
>   * %__GFP_ZERO returns a zeroed page on success.
>   *
>   * %__GFP_ZEROTAGS zeroes memory tags at allocation time if the
> memory itself
> - * is being zeroed (either via __GFP_ZERO or via init_on_alloc).
> + * is being zeroed (either via __GFP_ZERO or via init_on_alloc,
> provided that
> + * __GFP_SKIP_ZERO is not set).
> + *
> + * %__GFP_SKIP_ZERO makes page_alloc skip zeroing memory.
> + * Only effective when HW_TAGS KASAN is enabled.
>   *
>   * %__GFP_SKIP_KASAN_UNPOISON makes KASAN skip unpoisoning on page
> allocation.
>   * Only effective in HW_TAGS mode.
> @@ -242,6 +247,7 @@ struct vm_area_struct;
>  #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
>  #define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)
>  #define __GFP_ZEROTAGS	((__force gfp_t)___GFP_ZEROTAGS)
> +#define __GFP_SKIP_ZERO ((__force gfp_t)___GFP_SKIP_ZERO)
>  #define __GFP_SKIP_KASAN_UNPOISON ((__force
> gfp_t)___GFP_SKIP_KASAN_UNPOISON)
>  #define __GFP_SKIP_KASAN_POISON   ((__force
> gfp_t)___GFP_SKIP_KASAN_POISON)
>  
> @@ -249,7 +255,7 @@ struct vm_area_struct;
>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>  
>  /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) -
> 1))
>  
>  /**
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f1d5b80591c4..af7516a2d5ea 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2409,10 +2409,21 @@ static inline bool
> should_skip_kasan_unpoison(gfp_t flags, bool init_tags)
>  	return init_tags || (flags & __GFP_SKIP_KASAN_UNPOISON);
>  }
>  
> +static inline bool should_skip_init(gfp_t flags)
> +{
> +	/* Don't skip if a software KASAN mode is enabled. */
> +	if (!IS_ENABLED(CONFIG_KASAN_HW_TAGS))
> +		return false;
> +

Hi Andrey,

Should we use kasan_hw_tags_enabled() in should_skip_init() function
instead of checking the config?

I think we should handle the condition which is CONFIG_KASAN_HW_TAGS=y
and command line="kasan=off".


> +	/* For hardware tag-based KASAN, skip if requested. */
> +	return (flags & __GFP_SKIP_ZERO);
> +}
> +
>  inline void post_alloc_hook(struct page *page, unsigned int order,
>  				gfp_t gfp_flags)
>  {
> -	bool init = !want_init_on_free() &&
> want_init_on_alloc(gfp_flags);
> +	bool init = !want_init_on_free() &&
> want_init_on_alloc(gfp_flags) &&
> +			!should_skip_init(gfp_flags);
>  	bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
>  
>  	set_page_private(page, 0);
> -- 
> 2.25.1
> 
>