linux-kernel - Re: [PATCH 2/5] kernel/dma: remove unnecessary unmap_kernel

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Date:   Thu, 28 Jan 2021 09:44:15 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christoph Hellwig <hch@....de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 2/5] kernel/dma: remove unnecessary unmap_kernel_range

Excerpts from Christoph Hellwig's message of January 27, 2021 5:10 pm:
> On Tue, Jan 26, 2021 at 05:08:46PM -0500, Konrad Rzeszutek Wilk wrote:
>> On Tue, Jan 26, 2021 at 02:54:01PM +1000, Nicholas Piggin wrote:
>> > vunmap will remove ptes.
>> 
>> Should there be some ASSERT after the vunmap to make sure that is the
>> case? 
> 
> Not really.  removing the PTEs is the whole point of vunmap.  Everything
> else is just house keeping.

Agree. I did double check this and wrote a quick test to check ptes were 
there before the vunmap and cleared after, just to make sure I didn't 
make a silly mistake with the patch. But in general drivers should be 
able to trust code behind the API call will do the right thing. Such 
assertions should go in the vunmap() implementation as appropriate.

Thanks,
Nick