linux-kernel - Re: [PATCH 1/2] arm64, kdump: enforce to take 4G as the crashkernel low memory end

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Date:   Tue, 6 Sep 2022 15:05:57 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        guanghuifeng@...ux.alibaba.com, mark.rutland@....com,
        will@...nel.org, linux-mm@...ck.org, thunder.leizhen@...wei.com,
        wangkefeng.wang@...wei.com, kexec@...ts.infradead.org
Subject: Re: [PATCH 1/2] arm64, kdump: enforce to take 4G as the crashkernel
 low memory end

On Mon, 5 Sept 2022 at 14:08, Baoquan He <bhe@...hat.com> wrote:
>
> On 09/05/22 at 01:28pm, Mike Rapoport wrote:
> > On Thu, Sep 01, 2022 at 08:25:54PM +0800, Baoquan He wrote:
> > > On 09/01/22 at 10:24am, Mike Rapoport wrote:
> > >
> > > max_zone_phys() only handles cases when CONFIG_ZONE_DMA/DMA32 enabled,
> > > the disabledCONFIG_ZONE_DMA/DMA32 case is not included. I can change
> > > it like:
> > >
> > > static phys_addr_t __init crash_addr_low_max(void)
> > > {
> > >         phys_addr_t low_mem_mask = U32_MAX;
> > >         phys_addr_t phys_start = memblock_start_of_DRAM();
> > >
> > >         if ((!IS_ENABLED(CONFIG_ZONE_DMA) && !IS_ENABLED(CONFIG_ZONE_DMA32)) ||
> > >              (phys_start > U32_MAX))
> > >                 low_mem_mask = PHYS_ADDR_MAX;
> > >
> > >         return low_mem_mast + 1;
> > > }
> > >
> > > or add the disabled CONFIG_ZONE_DMA/DMA32 case into crash_addr_low_max()
> > > as you suggested. Which one do you like better?
> > >
> > > static phys_addr_t __init crash_addr_low_max(void)
> > > {
> > >         if (!IS_ENABLED(CONFIG_ZONE_DMA) && !IS_ENABLED(CONFIG_ZONE_DMA32))
> > >             return PHYS_ADDR_MAX + 1;
> > >
> > >         return max_zone_phys(32);
> > > }
> >
> > I like the second variant better.
>
> Sure, will change to use the 2nd one . Thanks.
>

While I appreciate the effort that has gone into solving this problem,
I don't think there is any consensus that an elaborate fix is required
to ensure that the crash kernel can be unmapped from the linear map at
all cost. In fact, I personally think we shouldn't bother, and IIRC,
Will made a remark along the same lines back when the Huawei engineers
were still driving this effort.

So perhaps we could align on that before doing yet another version of this?