linux-kernel - Re: [PATCH v3 3/3] arm64: mte: Simplify swap tag restoration logic

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Message-ID: <ZGepsWDEfG+gk/t3@arm.com>
Date:   Fri, 19 May 2023 17:54:09 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Peter Collingbourne <pcc@...gle.com>
Cc:     Qun-wei Lin (林群崴) 
        <Qun-wei.Lin@...iatek.com>, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "surenb@...gle.com" <surenb@...gle.com>,
        "david@...hat.com" <david@...hat.com>,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
        "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        Kuan-Ying Lee (李冠穎) 
        <Kuan-Ying.Lee@...iatek.com>,
        Casper Li (李中榮) <casper.li@...iatek.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        vincenzo.frascino@....com,
        Alexandru Elisei <alexandru.elisei@....com>, will@...nel.org,
        eugenis@...gle.com, Steven Price <steven.price@....com>
Subject: Re: [PATCH v3 3/3] arm64: mte: Simplify swap tag restoration logic

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@....com>