linux-kernel - Re: [PATCH] ARM: DMA: Fix kzalloc flags in __iommu_alloc

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Date:	Tue, 9 Dec 2014 11:57:22 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Alexandre Courbot <acourbot@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH] ARM: DMA: Fix kzalloc flags in __iommu_alloc_buffer()

On Mon, Dec 8, 2014 at 7:24 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday 08 December 2014 17:39:27 Alexandre Courbot wrote:
>> There doesn't seem to be any valid reason to allocate the pages array
>> with the same flags as the buffer itself. Doing so can eventually lead
>> to the following safeguard in mm/slab.c to be hit:
>>
>> BUG_ON(flags & GFP_SLAB_BUG_MASK);
>>
>> This happens when buffers are allocated with __GFP_DMA32 or
>> __GFP_HIGHMEM.
>>
>> Fix this by allocating the pages array with GFP_KERNEL to follow what is
>> done elsewhere in this file. Using GFP_KERNEL in __iommu_alloc_buffer()
>> is safe because atomic allocations are handled by __iommu_alloc_atomic().
>>
>
> I think you need to carry over the GFP_ATOMIC flag if that is set by the
> caller, but not the GFP_HIGHMEM or GFP_DMA32. Not sure if it's better
> to mask out flags from the caller mask, or to start with GFP_KERNEL
> and adding in extra bits.

I thought the issue of atomicity is already handled by
__iommu_alloc_buffer's caller (arm_iommu_alloc_attrs):

    if (!(gfp & __GFP_WAIT))
        return __iommu_alloc_atomic(dev, size, handle);
    ....
    pages = __iommu_alloc_buffer(dev, size, gfp, attrs);

Isn't the interesting property about GFP_ATOMIC that it does not
include __GFP_WAIT? I may very well misunderstand the issue, sorry if
that's the case.
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