linux-kernel - Re: [PATCH v4 2/3] common: DMA-mapping: add DMA_ATTR

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Message-ID: <20160108134201.GA20665@infradead.org>
Date:	Fri, 8 Jan 2016 05:42:01 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Douglas Anderson <dianders@...omium.org>
Cc:	Russell King <linux@....linux.org.uk>,
	Robin Murphy <robin.murphy@....com>,
	Tomasz Figa <tfiga@...omium.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Pawel Osciak <pawel@...iak.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>, corbet@....net,
	akpm@...ux-foundation.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] common: DMA-mapping: add DMA_ATTR_NOHUGEPAGE
 attribute

On Thu, Jan 07, 2016 at 04:36:44PM -0800, Douglas Anderson wrote:
> This patch adds the DMA_ATTR_NOHUGEPAGE attribute to the DMA-mapping
> subsystem.
> 
> This attribute can be used as a hint to the DMA-mapping subsystem that
> it's likely not worth it to try to allocation large pages behind the
> scenes.  Large pages are likely to make an IOMMU TLB work more
> efficiently but may not be worth it.  See the Documentation contained in
> this patch for more details about this attribute and when to use it.
> 
> Note that the name of the hint (DMA_ATTR_NOHUGEPAGE) is based on the
> name MADV_NOHUGEPAGE, which has the same meaning.  If we have expected
> users, we could also add MADV_HUGEPAGE which has the opposite meaning of
> this hint.

A user of this features seems to be missing in the series.  Please don't
add any clutter with unclear usage to the kernel unless there is a real
need which can be deonstrated in form of patches and numbers.