Re: [PATCH] ibmvscsi: use GFP_ATOMIC with dma_alloc_coherent in map_sg_data

[Tyrel Datwyler <tyreld@linux.vnet.ibm.com>]

On 01/10/2019 07:07 AM, Christoph Hellwig wrote:
> On Wed, Jan 09, 2019 at 06:58:56PM -0800, Tyrel Datwyler wrote:
>> While mapping DMA for scatter list when a scsi command is queued the
>> existing call to dma_alloc_coherent() in our map_sg_data() function
>> passes zero for the gfp_flags parameter. We are most definitly in atomic
>> context at this point as queue_command() is called in softirq context
>> and further we have a spinlock holding the scsi host lock.
>>
>> Fix this by passing GFP_ATOMIC to dma_alloc_coherent() to prevent any
>> sort of sleeping in atomic context deadlock.
> 
> This is a pretty clear sign you should not be using dma_alloc_coherent
> to start with.  GFP_ATOMIC support in many of the implementations either
> doesn't work at all or is severly constrained.  

On a secondary note I was unaware of the GFP_ATOMIC limitations. Should this be
added to the documentation somewhere? I don't see any mention here form
DMA-API-HOWTO.txt.

Using Consistent DMA mappings
=============================

To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
you should do::

        dma_addr_t dma_handle;

        cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);

where device is a ``struct device *``. This may be called in interrupt
context with the GFP_ATOMIC flag.

-Tyrel

Given that the
> descriptor is written by the OS and read by the hardware exactly once
> there is no point in having the coherent mapping to start with.
>