Date: Fri, 14 Oct 2016 11:13:04 +0800
From: Dave Young <dyoung@redhat.com>
To: Baoquan He <bhe@redhat.com>, ats-kumagai@wm.jp.nec.com
Cc: Pratyush Anand <panand@redhat.com>, keescook@chromium.org,
        ats-kumagai@wm.jp.nec.com, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, thgarnie@google.com,
        takahiro.akashi@linaro.org, mingo@redhat.com,
        "Eric W. Biederman" <ebiederm@xmission.com>, hpa@zytor.com,
        tglx@linutronix.de, akpm@linux-foundation.org, tonli@redhat.com
Subject: Re: [PATCH] kexec: Export memory sections virtual addresses to
 vmcoreinfo
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On 10/13/16 at 04:53pm, Baoquan He wrote:
> Hi Pratyush,
> 
> On 10/12/16 at 02:39pm, Pratyush Anand wrote:
> > 
> > 
> > On Wednesday 12 October 2016 05:56 AM, Baoquan He wrote:
> > > > PAGE_OFFSET can be get via vaddr - paddr from elf pt_loads so only
> > > > > VMALLOC_BASE and VMEMMAP_BASE is necessary..
> > > Well, yes, I was wrong. I wrongly thought of kernel text virtual address
> > > when I wrote the reply
> > 
> > So, if you can get PAGE_OFFSET then, probably you do not need to know
> > anything else.
> > 
> > I think, we can simplify makedumpfile code, where we do not need to depend
> > on VMALLOC_START or VMEMMAP_START etc.
> > 
> > "If we know PAGE_OFFSET, we can read from swapper space. If we can read from
> > swapper space, then we can know PA of any kernel VA, whether it is VMALLOC,
> > or vmemmap or module or kernel text area."
> 
> Check makedumpfile code and re-think about this, it's really like you
> said, we can convert VA to PA by swapper_pg_dir or init_level4_pgt. But the
> reason why we have to involve VMALLOC_START and VMEMMAP_START is that in
> x86_64 direct mapping and kernel text mapping are all linear mapping.
> Linear mapping can let us do a very efficient translation from VA to
> PA. Especially for page filtering, we need get PA of mm related data.
> All of them need convert by swapper_pg_dir or init_level4_pgt, that's
> inefficient, imagine the current system usually own many Tera bytes of
> physical memory.
> 

Atsushi, what do you think about above concern?  Ideally we should do it
in userspace instead of add more symbols. Maybe do a test on large
memory machine is necessary.

> So here though we can pick up crash memory regions from elf program
> header of vmcore and calculate the PAGE_OFFSET, we still need
> VMALLOC_START and VMEMMAP_START.
> 
> Thanks
> Baoquan
> > 
> > 
> > In fact, I have cleanup patches for ARM64 [1], which take above approach and
> > get rid of need of VMALLOC_START or VMEMMAP_START etc. I will be sending
> > them upstream soon.
> > 
> > Probably, x86 can take the similar approach.
> > 
> > ~Pratyush
> > 
> > [1] https://github.com/pratyushanand/makedumpfile/blob/arm64_devel/arch/arm64.c#L228
> > 
> 
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Thanks
Dave