Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc_coherent() calls

[Jason Cooper <jason@lakedaemon.net>]

On Tue, Jan 15, 2013 at 09:50:20AM -0800, Greg KH wrote:
> On Tue, Jan 15, 2013 at 11:56:42AM -0500, Jason Cooper wrote:
> > Greg,
> > 
> > I've added you to the this thread hoping for a little insight into USB
> > drivers and their use of coherent and GFP_ATOMIC.  Am I barking up the
> > wrong tree by looking a the drivers?
> 
> I don't understand, which drivers are you referring to?  USB host
> controller drivers, or the "normal" drivers?

Sorry I wasn't clear, I was referring specifically to the usb dvb
drivers em28xx, drxk and dib0700.  These are the drivers reported to be
in heavy use when the error occurs.

sata_mv is also in use, however no other users of sata_mv have reported
problems.  Including myself. ;-)

> Most USB drivers use GFP_ATOMIC if they are creating memory during
> their URB callback path, as that is interrupt context.  But it
> shouldn't be all that bad, and the USB core hasn't changed in a while,
> so something else must be causing this.

Agreed, so I went and did more reading.  The key piece of the puzzle
that I was missing was in arch/arm/mm/dma-mapping.c 660-684.

/*
 * Allocate DMA-coherent memory space and return both the kernel
 * remapped
 * virtual and bus address for that space.
 */
void *arm_dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
                    gfp_t gfp, struct dma_attrs *attrs)
{
        pgprot_t prot = __get_dma_pgprot(attrs, pgprot_kernel);
        void *memory;

        if (dma_alloc_from_coherent(dev, size, handle, &memory))
                return memory;

        return __dma_alloc(dev, size, handle, gfp, prot, false,
                           __builtin_return_address(0));
}

static void *arm_coherent_dma_alloc(struct device *dev, size_t size,
        dma_addr_t *handle, gfp_t gfp, struct dma_attrs *attrs)
{
        pgprot_t prot = __get_dma_pgprot(attrs, pgprot_kernel);
        void *memory;

        if (dma_alloc_from_coherent(dev, size, handle, &memory))
                return memory;

        return __dma_alloc(dev, size, handle, gfp, prot, true,
                           __builtin_return_address(0));
}


My understanding of this code is that when a driver requests dma memory,
we will first try to alloc from the per-driver pool.  If that fails, we
will then attempt to allocate from the atomic_pool.

Once the atomic_pool is exhausted, we get the error:

  ERROR: 1024 KiB atomic DMA coherent pool is too small!
  Please increase it with coherent_pool= kernel parameter!

If my understanding is correct, one of the drivers (most likely one)
either asks for too small of a dma buffer, or is not properly
deallocating blocks from the per-device pool.  Either case leads to
exhaustion, and falling back to the atomic pool.  Which subsequently
gets wiped out as well.

Am I on the right track?

thx,

Jason.

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