linux-kernel - Re: [PATCH] dma-debug: Check for drivers mapping vmalloc addresses

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Message-ID: <20180921052537.GB12825@lst.de>
Date:   Fri, 21 Sep 2018 07:25:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH] dma-debug: Check for drivers mapping vmalloc addresses

On Thu, Sep 20, 2018 at 03:35:59PM -0700, Stephen Boyd wrote:
> I recently debugged a DMA mapping oops where a driver was trying to map
> a buffer returned from request_firmware() with dma_map_single(). Memory
> returned from request_firmware() is mapped into the vmalloc region and
> this isn't a valid region to map with dma_map_single() per the DMA
> documentation's "What memory is DMA'able?" section.
> 
> Unfortunately, we don't really check that in the DMA debugging code, so
> enabling DMA debugging doesn't help catch this problem. Let's add a new
> DMA debug function to check for a vmalloc address and print a warning if
> this happens. This makes it a little easier to debug these sorts of
> problems, instead of seeing odd behavior or crashes when drivers attempt
> to map the vmalloc space for DMA.

This looks sensible to me.  I wonder if we shouldn't just throw in
an unconditional

	WARN_ON_ONCE(is_vmalloc_addr(ptr)));

into dma_map_single_attrs, though.