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Date:   Mon, 24 Jan 2022 18:31:39 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: make 'highmem' symbol ro_after_init
Message-ID: <Ye7wiwq/KFS3/bqL@casper.infradead.org>
References: <20220124170555.1054480-1-ardb@kernel.org>
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On Mon, Jan 24, 2022 at 06:05:55PM +0100, Ard Biesheuvel wrote:
> The 'highmem' variable is only set at boot, so we can make it
> ro_after_init and prevent it from being corrupted inadvertently, or from
> ending up in a contended cacheline.

I'm not against this patch, but it'd be nice to go further and remove
it entirely for !CONFIG_HIGHMEM builds?  Adding something like

static inline bool is_high_pfn(unsigned long pfn)
{
#ifdef CONFIG_HIGHMEM
	return pfn <= max_low_pfn;
#else
	return false;
#endif
}

static inline bool is_high_phys(phys_addr_t pa)
{
	return is_high_pfn(pa / PAGE_SIZE);
}

static inline bool is_high_addr(void *addr)
{
	return is_high_phys(virt_to_phys(addr));
}

should be all the primitives you need, from a quick grep.

> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..50e82cb94ccc 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
>   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
>   * and ZONE_HIGHMEM.
>   */
> -void *high_memory;
> +void *high_memory __ro_after_init;
>  EXPORT_SYMBOL(high_memory);
>  
>  /*
> -- 
> 2.30.2
> 
>