linux-kernel - Re: [PATCH] arm: Extend the check for RAM in /dev/mem

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Date:   Fri, 12 Jul 2019 08:06:43 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     KarimAllah Ahmed <karahmed@...zon.de>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Enrico Weigelt <info@...ux.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Jun Yao <yaojun8558363@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] arm: Extend the check for RAM in /dev/mem



On 07/12/2019 03:51 AM, KarimAllah Ahmed wrote:
> Some valid RAM can live outside kernel control (e.g. using mem= kernel
> command-line). For these regions, pfn_valid would return "false" causing
> system RAM to be mapped as uncached. Use memblock instead to identify RAM.

Once the remaining memory is outside of the kernel (as the admin would have
intended with mem= command line) what is the particular concern regarding
the way those get mapped (cached or not) ? It is not to be used any way.

> 
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mike Rapoport <rppt@...ux.ibm.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Anders Roxell <anders.roxell@...aro.org>
> Cc: Enrico Weigelt <info@...ux.net>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: KarimAllah Ahmed <karahmed@...zon.de>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: James Morse <james.morse@....com>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Jun Yao <yaojun8558363@...il.com>
> Cc: Yu Zhao <yuzhao@...gle.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> ---
>  arch/arm/mm/mmu.c   | 2 +-
>  arch/arm64/mm/mmu.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 1aa2586..492774b 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -705,7 +705,7 @@ static void __init build_mem_type_table(void)
>  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  			      unsigned long size, pgprot_t vma_prot)
>  {
> -	if (!pfn_valid(pfn))
> +	if (!memblock_is_memory(__pfn_to_phys(pfn)))
>  		return pgprot_noncached(vma_prot);
>  	else if (file->f_flags & O_SYNC)
>  		return pgprot_writecombine(vma_prot);
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 3645f29..cdc3e8e 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -78,7 +78,7 @@ void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  			      unsigned long size, pgprot_t vma_prot)
>  {
> -	if (!pfn_valid(pfn))
> +	if (!memblock_is_memory(__pfn_to_phys(pfn)))

pfn_valid() on arm64 checks if the memblock region is mapped i.e does it have
a linear mapping or not. If a segment of RAM is outside linear mapping due to
mem= directive and lacks a linear mapping then why should it be mapped similarly
like system RAM on this path ?