From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Cc: Alexandre Courbot <acourbot@nvidia.com>,
        Russell King <linux@arm.linux.org.uk>,
        Marek Szyprowski <m.szyprowski@samsung.com>, gnurou@gmail.com,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        linux-kernel@vger.kernel.org, Thierry Reding <treding@nvidia.com>
Subject: Re: [PATCH] ARM: DMA: Fix kzalloc flags in __iommu_alloc_buffer()
Date: Mon, 08 Dec 2014 11:24 +0100
Message-ID: <10532762.PXKTQM8KjH@wuerfel>
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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.

	Arnd
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