linux-kernel - Re: [PATCH v4 0/3] dma-mapping, powerpc, nvme: introduce the DMA_ATTR_NO

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Date:	Thu, 4 Aug 2016 15:01:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>
Cc:	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org, corbet@....net,
	rmk+kernel@....linux.org.uk, keith.busch@...el.com, axboe@...com,
	benh@...nel.crashing.org, mpe@...erman.id.au,
	k.kozlowski@...sung.com
Subject: Re: [PATCH v4 0/3] dma-mapping, powerpc, nvme: introduce the
 DMA_ATTR_NO_WARN attribute

On Mon,  1 Aug 2016 19:59:47 -0300 Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com> wrote:

> This patchset introduces dma_attr DMA_ATTR_NO_WARN (just like __GFP_NOWARN),
> which tells the DMA-mapping subsystem to suppress allocation failure reports.
> 
> On some architectures allocation failures are reported with error messages
> to the system logs.  Although this can help to identify and debug problems,
> drivers which handle failures (eg, retry later) have no problems with them,
> and can actually flood the system logs with error messages that aren't any
> problem at all, depending on the implementation of the retry mechanism.

It would help to have seen an example of the error message - please
always quote such things when fixing bugs.

I assume the warnings are coming via nvme_map_data()'s call to
blk_rq_map_sg()?  An alternative (and more idiomatic) fix would be to
change the blk_rq_map_sg() interface to permit passing down some
foo_NOWARN flag and propagating that down the stack into
ppc_iommu_map_sg().  Was this approach evaluated?  I suspect it might
be messy.