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Date:   Mon, 4 May 2020 12:10:36 +0900
From:   Joonsoo Kim <>
To:     "Eric W. Biederman" <>
Cc:     Andrew Morton <>,
        Linux Memory Management List <>,
        LKML <>,
        Vlastimil Babka <>,
        Laura Abbott <>,
        "Aneesh Kumar K . V" <>,
        Mel Gorman <>,
        Michal Hocko <>,
        Johannes Weiner <>,
        Roman Gushchin <>, Minchan Kim <>,
        Rik van Riel <>,
        Christian Koenig <>,
        Huang Rui <>,
        "Rafael J . Wysocki" <>,
        Pavel Machek <>,,
        Christoph Hellwig <>,
        Joonsoo Kim <>
Subject: Re: [PATCH v2 03/10] kexec: separate PageHighMem() and
 PageHighMemZone() use case

2020년 5월 1일 (금) 오후 11:06, Eric W. Biederman <>님이 작성:
> writes:
> > From: Joonsoo Kim <>
> >
> > 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 #2 for this patch.
> Hmm.
> What happened to the notion of deprecating and reducing the usage of
> highmem?  I know that we have some embedded architectures where it is
> still important but this feels like it flies in the face of that.

AFAIK, deprecating highmem requires some more time and, before then,
we need to support it.

> This part of kexec would be much more maintainable if it had a proper
> mm layer helper that tested to see if the page matched the passed in
> gfp flags.  That way the mm layer could keep changing and doing weird
> gyrations and this code would not care.

Good idea! I will do it.

> What would be really helpful is if there was a straight forward way to
> allocate memory whose physical address fits in the native word size.
> All I know for certain about this patch is that it takes a piece of code
> that looked like it made sense, and transfroms it into something I can
> not easily verify, and can not maintain.

Although I decide to make a helper as you described above, I don't
understand why you think that a new code isn't maintainable. It is just
the same thing with different name. Could you elaborate more why do
you think so?


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