linux-kernel - Re: [PATCH v2 03/10] kexec: separate PageHighMem() and PageHighMemZone() use case

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Date:   Mon, 4 May 2020 12:10:36 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Laura Abbott <labbott@...hat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, kernel-team@....com,
        Christoph Hellwig <hch@...radead.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 03/10] kexec: separate PageHighMem() and
 PageHighMemZone() use case

2020년 5월 1일 (금) 오후 11:06, Eric W. Biederman <ebiederm@...ssion.com>님이 작성:
>
> js1304@...il.com writes:
>
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> >
> > 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?

Thanks.