linux-kernel - Re: [PATCH] ARM:mm: fix kmap_atomic_to

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Date:	Mon, 12 Oct 2015 14:30:39 +0900
From:	Jongsung Kim <neidhard.kim@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Chanho Min <chanho.min@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Gunho Lee <gunho.lee@....com>
Subject: Re: [PATCH] ARM:mm: fix kmap_atomic_to_page

We tried to utilize a HW compressor as a zram backend. Current zram uses kmap_atomic to map a page, and the HW DMAes. So we needed to use kmap_atomic_to_page to get the page to be dma-mapped.


On 10/07/2015 06:01 PM, Russell King - ARM Linux wrote:
> On Wed, Oct 07, 2015 at 12:55:08PM +0900, Jongsung Kim wrote:
>> Recently, we made a driver utilizing kmap_atomic_to_page. Of course,
>> it's not mainlined. People may be using it outside mainline just like us.
> Since kmap_atomic() mappings are supposed to be short-lived, why do you
> need it in your driver?  Don't you already have the struct page pointer
> when setting up the kmap_atomic() mapping?
>
> It is invalid to setup a mapping, and leave it setup across any context
> switching or similar.
>
> Also, kmap_atomic_to_page() is not exported to modules, so you can only
> use it when built-in.
>
>> vmalloc has vmalloc_to_page, pkmap has kmap_to page, and fixmap has
>> kmap_atomic_to_page. Then.. how about letting virt_to_page do them all?
> No.  virt_to_page() is defined to only work on the lowmem mapping, and
> that's not going to change.
>
> Please show the outline of your code making use of this function so we
> can better understand your use case.
>

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