Re: [PATCH v2 06/10] mm/hugetlb: separate PageHighMem() and PageHighMemZone() use case

[Christoph Hellwig]

On Wed, Apr 29, 2020 at 12:26:39PM +0900, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> Until now, PageHighMem() is used for two different cases. One is to check
> if there is a direct mapping for this page or not. The other is to check
> the zone of this page, that is, weather it is the highmem type zone or not.
> 
> Now, we have separate functions, PageHighMem() and PageHighMemZone() for
> each cases. Use appropriate one.
> 
> Note that there are some rules to determine the proper macro.
> 
> 1. If PageHighMem() is called for checking if the direct mapping exists
> or not, use PageHighMem().
> 2. If PageHighMem() is used to predict the previous gfp_flags for
> this page, use PageHighMemZone(). The zone of the page is related to
> the gfp_flags.
> 3. If purpose of calling PageHighMem() is to count highmem page and
> to interact with the system by using this count, use PageHighMemZone().
> This counter is usually used to calculate the available memory for an
> kernel allocation and pages on the highmem zone cannot be available
> for an kernel allocation.
> 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation
> is just copy of the previous PageHighMem() implementation and won't
> be changed.
> 
> I apply the rule #3 for this patch.
> 
> Acked-by: Roman Gushchin <guro@xxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Why do we care about the zone here?  This only cares about having
kernel direct mapped pages as far as I can tell.