linux-kernel - Re: [PATCH v3 1/4] net

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Date:	Wed, 15 Jan 2014 13:33:52 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	saeed bishara <saeed.bishara@...il.com>
Cc:	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Whipple <whipple@...uredatainnovations.ch>,
	lkml <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3 1/4] net_dma: simple removal

On Wed, Jan 15, 2014 at 1:31 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Wed, Jan 15, 2014 at 1:20 PM, saeed bishara <saeed.bishara@...il.com> wrote:
>> Hi Dan,
>>
>> I'm using net_dma on my system and I achieve meaningful performance
>> boost when running Iperf receive.
>>
>> As far as I know the net_dma is used by many embedded systems out
>> there and might effect their performance.
>> Can you please elaborate on the exact scenario that cause the memory corruption?
>>
>> Is the scenario mentioned here caused by "real life" application or
>> this is more of theoretical issue found through manual testing, I was
>> trying to find the thread describing the failing scenario and couldn't
>> find it, any pointer will be appreciated.
>
> Did you see the referenced commit?
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=77873803363c
>
> This is a real issue in that any app that forks() while receiving data
> can cause the dma data to be lost.  The problem is that the copy
> operation falls back to cpu at many locations.  Any one of those
> instance could touch a mapped page and trigger a copy-on-write event.
> The dma completes to the wrong location.
>

Btw, do you have benchmark data showing that NET_DMA is beneficial on
these platforms?  I would have expected worse performance on platforms
without i/o coherent caches.
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