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Date:   Fri, 7 Jun 2019 19:30:16 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Dave Young <dyoung@redhat.com>, Pingfan Liu <kernelfans@gmail.com>
Cc:     kexec@lists.infradead.org, Baoquan He <bhe@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>, yinghai@kernel.org,
        vgoyal@redhat.com, Randy Dunlap <rdunlap@infradead.org>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr
Message-ID: <20190607173016.GM20269@zn.tnic>
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On Mon, Jan 28, 2019 at 11:18:31AM +0100, Borislav Petkov wrote:
> On Mon, Jan 28, 2019 at 05:58:09PM +0800, Dave Young wrote:
> > Another reason is in case ,high we will need automatically reserve a
> > region in low area for swiotlb.  So for example one use
> > crashkernel=256M,high,  actual reserved memory is 256M above 4G and
> > another 256M under 4G for swiotlb.  Normally it is not necessary for
> > most people.  Thus we can not make ,high as default.
> 
> And how is the poor user to figure out that we decided for her/him that
> swiotlb reservation is something not necessary for most people and thus
> we fail the crashkernel= reservation?
> 
> IOW, that "logic" above doesn't make a whole lot of sense to me from
> user friendliness perspective.

So to show what I mean: I'm trying to reserve a crash kernel region on a
box here. I tried:

crashkernel=64M@16M

as it is stated in Documentation/kdump/kdump.txt.

Box said:

[    0.000000] crashkernel reservation failed - memory is in use.

Oh great.

Then I tried:

crashkernel=64M@64M

Box said:

[    0.000000] crashkernel reservation failed - memory is in use.

So I simply did:

crashkernel=64M

and the box said:

[    0.000000] Reserving 64MB of memory at 3392MB for crashkernel (System RAM: 16271MB)

So I could've gone a long time poking at the memory to find a suitable
address.

So do you see what I mean with making this as user-friendly and as
robust as possible?

In this case I don't care about *where* my crash kernel is - I only want
to have one loaded *somewhere*.

And the same strategy should be applied to other reservation attempts
- we should try hard to reserve and if we cannot reserve, then try an
alternating range.

I even think that

crashkernel=X@Y

should not simply fail if Y is occupied but keep trying and say

[    0.000000] Reserving 64MB of memory at alternative address 3392MB for crashkernel (System RAM: 16271MB)

and only fail when the user doesn't really want the kernel to try hard
by booting with

crashkernel=X@Y,strict

But that's for another day.

-- 
Regards/Gruss,
    Boris.

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