Messages in this thread | | | Date | Tue, 28 Oct 2014 12:08:46 -0700 | From | Florian Fainelli <> | Subject | DMA allocations from CMA and fatal_signal_pending check |
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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.
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
Now the general problem that I see with this fatal_signal_pending() check is that any driver that calls dma_alloc_coherent() and which does this in a process context (network drivers are frequently doing this in their ndo_open callback) and also happens to get its allocation serviced from CMA can now fail, instead of failing on really hard OOM conditions. -- Florian
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