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Date:   Fri, 4 Sep 2020 12:16:33 +0800
From:   Dave Young <dyoung@redhat.com>
To:     chenzhou <chenzhou10@huawei.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>, will@kernel.org,
        james.morse@arm.com, tglx@linutronix.de, mingo@redhat.com,
        bhe@redhat.com, corbet@lwn.net, John.P.donnelly@oracle.com,
        prabhakar.pkin@gmail.com, bhsharma@redhat.com, horms@verge.net.au,
        robh+dt@kernel.org, arnd@arndb.de, nsaenzjulienne@suse.de,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kexec@lists.infradead.org, linux-doc@vger.kernel.org,
        guohanjun@huawei.com, xiexiuqi@huawei.com, huawei.libin@huawei.com,
        wangkefeng.wang@huawei.com
Subject: Re: [PATCH v11 3/5] arm64: kdump: reimplement crashkernel=X
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On 09/04/20 at 12:02pm, chenzhou wrote:
> 
> 
> On 2020/9/4 11:10, Dave Young wrote:
> > On 09/04/20 at 11:04am, Dave Young wrote:
> >> On 09/03/20 at 07:26pm, chenzhou wrote:
> >>> Hi Catalin,
> >>>
> >>>
> >>> On 2020/9/3 1:09, Catalin Marinas wrote:
> >>>> On Sat, Aug 01, 2020 at 09:08:54PM +0800, Chen Zhou wrote:
> >>>>> There are following issues in arm64 kdump:
> >>>>> 1. We use crashkernel=X to reserve crashkernel below 4G, which
> >>>>> will fail when there is no enough low memory.
> >>>>> 2. If reserving crashkernel above 4G, in this case, crash dump
> >>>>> kernel will boot failure because there is no low memory available
> >>>>> for allocation.
> >>>>> 3. Since commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32"),
> >>>>> if the memory reserved for crash dump kernel falled in ZONE_DMA32,
> >>>>> the devices in crash dump kernel need to use ZONE_DMA will alloc
> >>>>> fail.
> >>>>>
> >>>>> To solve these issues, change the behavior of crashkernel=X.
> >>>>> crashkernel=X tries low allocation in ZONE_DMA, and fall back to
> >>>>> high allocation if it fails.
> >>>>>
> >>>>> If requized size X is too large and leads to very little free memory
> >>>>> in ZONE_DMA after low allocation, the system may not work normally.
> >>>>> So add a threshold and go for high allocation directly if the required
> >>>>> size is too large. The value of threshold is set as the half of
> >>>>> the low memory.
> >>>>>
> >>>>> If crash_base is outside ZONE_DMA, try to allocate at least 256M in
> >>>>> ZONE_DMA automatically. "crashkernel=Y,low" can be used to allocate
> >>>>> specified size low memory.
> >>>> Except for the threshold to keep zone ZONE_DMA memory,
> >>>> reserve_crashkernel() looks very close to the x86 version. Shall we try
> >>>> to make this generic as well? In the first instance, you could avoid the
> >>>> threshold check if it takes an explicit ",high" option.
> >>> Ok, i will try to do this.
> >>>
> >>> I look into the function reserve_crashkernel() of x86 and found the start address is
> >>> CRASH_ALIGN in function memblock_find_in_range(), which is different with arm64.
> >>>
> >>> I don't figure out why is CRASH_ALIGN in x86, is there any specific reason?
> >> Hmm, took another look at the option CONFIG_PHYSICAL_ALIGN
> >> config PHYSICAL_ALIGN
> >>         hex "Alignment value to which kernel should be aligned"
> >>         default "0x200000"
> >>         range 0x2000 0x1000000 if X86_32
> >>         range 0x200000 0x1000000 if X86_64
> >>
> >> According to above, I think the 16M should come from the largest value
> >> But the default value is 2M,  with smaller value reservation can have
> >> more chance to succeed.
> >>
> >> It seems we still need arch specific CRASH_ALIGN, but the initial
> >> version you added the #ifdef for different arches, can you move the
> >> macro to arch specific headers?
> > And just keep the x86 align value as is, I can try to change the x86
> > value later to CONFIG_PHYSICAL_ALIGN, in this way this series can be
> > cleaner.
> Ok. I have no question about the value of macro CRASH_ALIGN,
> instead the lower bound of memblock_find_in_range().
> 
> For x86, in reserve_crashkernel()，restrict the lower bound of the range to CRASH_ALIGN,
>     ...
>     crash_base = memblock_find_in_range(CRASH_ALIGN,
>                                                 CRASH_ADDR_LOW_MAX,
>                                                 crash_size, CRASH_ALIGN);
>     ...
>    
> in reserve_crashkernel_low()，with no this restriction.
>     ...
>     low_base = memblock_find_in_range(0, 1ULL << 32, low_size, CRASH_ALIGN);
>     ...
> 
> How about all making memblock_find_in_range() search from the start of memory?
> If it is ok, i will do like this in the generic version.

I feel starting with CRASH_ALIGN sounds better, can you just search from
CRASH_ALIGN in generic version?

Thanks
Dave