[PATCH v3 3/3] arm64: mte: Simplify swap tag restoration logic

[Catalin Marinas]

On Tue, May 16, 2023 at 07:21:13PM -0700, Peter Collingbourne wrote:
> As a result of the previous two patches, there are no circumstances
> in which a swapped-in page is installed in a page table without first
> having arch_swap_restore() called on it. Therefore, we no longer need
> the logic in set_pte_at() that restores the tags, so remove it.
> 
> Because we can now rely on the page being locked, we no longer need to
> handle the case where a page is having its tags restored by multiple tasks
> concurrently, so we can slightly simplify the logic in mte_restore_tags().
[...]
> diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
> index cd508ba80ab1..3a78bf1b1364 100644
> --- a/arch/arm64/mm/mteswap.c
> +++ b/arch/arm64/mm/mteswap.c
> @@ -53,10 +53,9 @@ void mte_restore_tags(swp_entry_t entry, struct page *page)
>  	if (!tags)
>  		return;
>  
> -	if (try_page_mte_tagging(page)) {
> -		mte_restore_page_tags(page_address(page), tags);
> -		set_page_mte_tagged(page);
> -	}
> +	WARN_ON_ONCE(!try_page_mte_tagging(page));
> +	mte_restore_page_tags(page_address(page), tags);
> +	set_page_mte_tagged(page);
>  }

Can we have a situation where two processes share the same swap pte
(CoW) and they both enter the do_swap_page() or the unuse_pte() paths
triggering this warning?

Other than that, the looks nice, it simplifies the logic and probably
saves a few cycles as well on the set_pte_at() path.

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>