linux-kernel - Re: [PATCH] arm: xen: mm: use __GPF

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Date:   Tue, 27 Aug 2019 13:23:22 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Peng Fan <peng.fan@....com>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "van.freenix@...il.com" <van.freenix@...il.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm: xen: mm: use __GPF_DMA32 for arm64

On 09/07/2019 09:22, Peng Fan wrote:
> arm64 shares some code under arch/arm/xen, including mm.c.
> However ZONE_DMA is removed by commit
> ad67f5a6545("arm64: replace ZONE_DMA with ZONE_DMA32").
> So to ARM64, need use __GFP_DMA32.
> 
> Signed-off-by: Peng Fan <peng.fan@....com>
> ---
>   arch/arm/xen/mm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
> index e1d44b903dfc..a95e76d18bf9 100644
> --- a/arch/arm/xen/mm.c
> +++ b/arch/arm/xen/mm.c
> @@ -27,7 +27,7 @@ unsigned long xen_get_swiotlb_free_pages(unsigned int order)
>   
>   	for_each_memblock(memory, reg) {
>   		if (reg->base < (phys_addr_t)0xffffffff) {
> -			flags |= __GFP_DMA;
> +			flags |= __GFP_DMA | __GFP_DMA32;

Given the definition of GFP_ZONE_BAD, I'm not sure this combination of 
flags is strictly valid, but rather is implicitly reliant on only one of 
those zones ever actually existing. As such, it seems liable to blow up 
if the plans to add ZONE_DMA to arm64[1] go ahead.

Robin.

[1] 
https://lore.kernel.org/linux-arm-kernel/20190820145821.27214-1-nsaenzjulienne@suse.de/

>   			break;
>   		}
>   	}
>