Re: [PATCH 3/3] vmalloc: hw_tags: optimize vmalloc redzoning

[Dev Jain <dev.jain@arm.com>]

On 13/05/26 4:27 pm, Dev Jain wrote:
> If the allocation size is less than a page, vmalloc first unpoisons the
> entire page, then poisons the tail with KASAN_TAG_INVALID (for hw tags),
> to catch OOB.
> 
> Instead, unpoison the allocation length, and then poison the tail,
> saving some work.


My bad, I see that the actual allocation size (and not page-aligned size)
is passed down to kasan_unpoison_vmalloc, so this patch is a no-op.

I'll drop it.


> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/kasan/hw_tags.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index cbef5e450954..7c94f71b5f12 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -364,9 +364,6 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
>  	tag = (flags & KASAN_VMALLOC_KEEP_TAG) ? get_tag(start) : kasan_random_tag();
>  	start = set_tag(start, tag);
>  
> -	/* Unpoison and initialize memory up to size. */
> -	kasan_unpoison(start, size, flags & KASAN_VMALLOC_INIT);
> -
>  	/*
>  	 * Explicitly poison and initialize the in-page vmalloc() redzone.
>  	 * Unlike software KASAN modes, hardware tag-based KASAN doesn't
> @@ -375,8 +372,14 @@ void *__kasan_unpoison_vmalloc(const void *start, unsigned long size,
>  	redzone_start = round_up((unsigned long)start + size,
>  				 KASAN_GRANULE_SIZE);
>  	redzone_size = round_up(redzone_start, PAGE_SIZE) - redzone_start;
> -	kasan_poison((void *)redzone_start, redzone_size, KASAN_TAG_INVALID,
> -		     flags & KASAN_VMALLOC_INIT);
> +
> +	/* Unpoison and initialize memory before the redzone. */
> +	kasan_unpoison(start, redzone_start - (unsigned long)start,
> +		       flags & KASAN_VMALLOC_INIT);
> +
> +	if (redzone_size)
> +		kasan_poison((void *)redzone_start, redzone_size,
> +			     KASAN_TAG_INVALID, flags & KASAN_VMALLOC_INIT);
>  
>  	/*
>  	 * Set per-page tag flags to allow accessing physical memory for the