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Date:   Thu, 15 Sep 2022 13:58:38 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexei Starovoitov <ast@kernel.org>,
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Subject: Re: [PATCH v7 27/43] kmsan: disable physical page merging in biovec
Message-Id: <20220915135838.8ad6df0363ccbd671d9641a1@linux-foundation.org>
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        <20220915150417.722975-28-glider@google.com>
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On Thu, 15 Sep 2022 17:04:01 +0200 Alexander Potapenko <glider@google.com> wrote:

> KMSAN metadata for adjacent physical pages may not be adjacent,
> therefore accessing such pages together may lead to metadata
> corruption.
> We disable merging pages in biovec to prevent such corruptions.
> 
> ...
>
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -88,6 +88,13 @@ static inline bool biovec_phys_mergeable(struct request_queue *q,
>  	phys_addr_t addr1 = page_to_phys(vec1->bv_page) + vec1->bv_offset;
>  	phys_addr_t addr2 = page_to_phys(vec2->bv_page) + vec2->bv_offset;
>  
> +	/*
> +	 * Merging adjacent physical pages may not work correctly under KMSAN
> +	 * if their metadata pages aren't adjacent. Just disable merging.
> +	 */
> +	if (IS_ENABLED(CONFIG_KMSAN))
> +		return false;
> +
>  	if (addr1 + vec1->bv_len != addr2)
>  		return false;
>  	if (xen_domain() && !xen_biovec_phys_mergeable(vec1, vec2->bv_page))

What are the runtime effects of this?  In other words, how much
slowdown is this likely to cause in a reasonable worst-case?