DMA allocations from CMA and fatal_signal_pending check

[Joonsoo Kim]

On Tue, Oct 28, 2014 at 12:08:46PM -0700, Florian Fainelli wrote:
> Hello,
> 
> While debugging why some dma_alloc_coherent() allocations where
> returning NULL on our brcmstb platform, specifically with
> drivers/net/ethernet/broadcom/bcmcsysport.c, I came across the
> fatal_signal_pending() check in mm/page_alloc.c which is there.
> 
> This driver calls dma_alloc_coherent(, GFP_KERNEL) which ends up making
> a coherent allocation from a CMA region on our platform. Since that
> allocation is allowed to sleep, and because we are in bcm_syport_open(),
> executed from process context, a pending signal makes
> dma_alloc_coherent() return NULL.

Hello, Florian.

fatal_signal_pending means that there is SIGKILL on that process.
I guess that caller of dma_alloc_coherent() will die soon.
In this case, why CMA should be succeed?

> 
> There are two ways I could fix this:
> 
> - use a GFP_ATOMIC allocation, which would avoid this sensitivity to a
> pending signal being fatal (we suffer from the same issue in
> bcm_sysport_resume)
> 
> - move the DMA coherent allocation before bcm_sysport_open(), in the
> driver's probe function, but if the network interface is never used, we
> would be waisting precious DMA coherent memory for nothing (it is only 4
> bytes times 32 but still

I guess that it is okay that bcm_sysport_open() return -EINTR?

Thanks.