linux-kernel - Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

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Date:   Mon, 25 Feb 2019 09:53:46 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Borislav Petkov <bp@...en8.de>, Joerg Roedel <jroedel@...e.de>,
        Baoquan He <bhe@...hat.com>,
        Jerry Hoemann <jerry.hoemann@....com>, x86@...nel.org,
        Randy Dunlap <rdunlap@...radead.org>,
        kexec@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yinghai Lu <yinghai@...nel.org>, vgoyal@...hat.com,
        iommu@...ts.linux-foundation.org, konrad.wilk@...cle.com
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr

On 02/24/19 at 09:25pm, Pingfan Liu wrote:
> On Fri, Feb 22, 2019 at 9:00 PM Borislav Petkov <bp@...en8.de> wrote:
> >
> > On Fri, Feb 22, 2019 at 09:42:41AM +0100, Joerg Roedel wrote:
> > > The current default of 256MB was found by experiments on a bigger
> > > number of machines, to create a reasonable default that is at least
> > > likely to be sufficient of an average machine.
> >
> > Exactly, and this is what makes sense.
> >
> > The code should try the requested reservation and if it fails, it should
> > try high allocation with default swiotlb size because we need to reserve
> > *some* range.
> >
> > If that reservation succeeds, we should say something along the lines of
> >
> > "... requested range failed, reserved <X> range instead."
> >
> Maybe I misunderstood you, but does "requested range failed" mean that
> user specify the range? If yes, then it should be the duty of user as
> you said later, not the duty of kernel"

If you go with the changes in your current patch it is needed to say
something like:
"crashkernel: can not find free memory under 4G, reserve XM@.. instead" 

Also need to print the reserved low memory area in case ,high being used.

But for 896M -> 4G, the 896M faulure is not necessary to show in dmesg,
it is some in kernel logic.

Thanks
Dave