netdev - DMA allocations from CMA and fatal_signal

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Message-ID: <544FE9BE.6040503@gmail.com>
Date:	Tue, 28 Oct 2014 12:08:46 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	linux-arm-kernel@...ts.infradead.org,
	Brian Norris <computersforpeace@...il.com>,
	Gregory Fong <gregory.0xf0@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lauraa@...eaurora.org, gioh.kim@....com,
	aneesh.kumar@...ux.vnet.ibm.com, iamjoonsoo.kim@....com,
	mina86@...a86.com, m.szyprowski@...sung.com,
	akpm@...ux-foundation.org,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: DMA allocations from CMA and fatal_signal_pending check

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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