Re: [PATCH v11 3/5] arm64: kdump: reimplement crashkernel=X

[Dave Young <dyoung@redhat.com>]

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.

> 
> Thanks
> Dave