linux-kernel - Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

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Date:   Tue, 6 Nov 2018 14:39:26 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>,
        Nicolin Chen <nicoleotsuka@...il.com>
Cc:     joro@...tes.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

On 05/11/2018 14:58, Christoph Hellwig wrote:
> On Fri, Nov 02, 2018 at 04:36:13PM -0700, Nicolin Chen wrote:
>>> What if the pages came from highmem? I know that doesn't happen on arm64
>>> today, but the point of this code *is* to be generic, and other users will
>>> arrive eventually.
>>
>> Hmm, so it probably should use sg_miter_start/stop() too? Looking
>> at the flush routine doing in PAGE_SIZE for each iteration, would
>> be possible to map and memset contiguous pages together? Actually
>> the flush routine might be also optimized if we can map contiguous
>> pages.
> 
> FYI, I have patches I plan to submit soon that gets rid of the
> struct scatterlist use in this code to simplify it:

...and I have some significant objections to that simplification which I 
plan to respond with ;)

(namely that it defaults the whole higher-order page allocation business 
which will have varying degrees of performance impact on certain cases)

Robin.