linux-kernel - Re: DMA allocations from CMA and fatal_signal

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Date:	Mon, 03 Nov 2014 17:45:31 +0100
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Florian Fainelli <f.fainelli@...il.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	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, m.szyprowski@...sung.com,
	akpm@...ux-foundation.org,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>
Subject: Re: DMA allocations from CMA and fatal_signal_pending check

On Fri, Oct 31 2014, Florian Fainelli wrote:
> I agree that the CMA allocation should not be allowed to succeed, but
> the dma_alloc_coherent() allocation should succeed. If we look at the
> sysport driver, there are kmalloc() calls to initialize private
> structures, those will succeed (except under high memory pressure), so
> by the same token, a driver expects DMA allocations to succeed (unless
> we are under high memory pressure)
>
> What are we trying to solve exactly with the fatal_signal_pending()
> check here? Are we just optimizing for the case where a process has
> allocated from a CMA region to allow this region to be returned to the
> pool of free pages when it gets killed? Could there be another mechanism
> used to reclaim those pages if we know the process is getting killed
> anyway?

We're guarding against situations where process may hang around
arbitrarily long time after receiving SIGKILL.  If user does “kill -9
$pid” the usual expectation is that the $pid process will die within
seconds and anything longer is perceived by user as a bug.

What problem are *you* trying to solve?  If user sent SIGKILL to
a process that imitated device initialisation, what is the point of
continuing initialising the device?  Just recover and return -EINTR.

> Well, not really. This driver is not an isolated case, there are tons of
> other networking drivers that do exactly the same thing, and we do
> expect these dma_alloc_* calls to succeed.

Again, why do you expect them to succeed?  The code must handle failures
correctly anyway so why do you wish to ignore fatal signal?

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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