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

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Message-ID: <20151007090121.GC21513@n2100.arm.linux.org.uk>
Date:	Wed, 7 Oct 2015 10:01:21 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jongsung Kim <neidhard.kim@....com>
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

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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