Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory

[Baoquan He]

On 06/21/22 at 02:24pm, Kefeng Wang wrote:
> 
> On 2022/6/21 13:33, Baoquan He wrote:
> > Hi,
> > 
> > On 06/13/22 at 04:09pm, Zhen Lei wrote:
> > > If the crashkernel has both high memory above DMA zones and low memory
> > > in DMA zones, kexec always loads the content such as Image and dtb to the
> > > high memory instead of the low memory. This means that only high memory
> > > requires write protection based on page-level mapping. The allocation of
> > > high memory does not depend on the DMA boundary. So we can reserve the
> > > high memory first even if the crashkernel reservation is deferred.
> > > 
> > > This means that the block mapping can still be performed on other kernel
> > > linear address spaces, the TLB miss rate can be reduced and the system
> > > performance will be improved.
> > Ugh, this looks a little ugly, honestly.
> > 
> > If that's for sure arm64 can't split large page mapping of linear
> > region, this patch is one way to optimize linear mapping. Given kdump
> > setting is necessary on arm64 server, the booting speed is truly
> > impacted heavily.
> 
> Is there some conclusion or discussion that arm64 can't split large page
> mapping?

Yes, please see below commit log. 
commit d27cfa1fc823 ("arm64: mm: set the contiguous bit for kernel mappings where appropriate")

> 
> Could the crashkernel reservation (and Kfence pool) be splited dynamically?

For crashkernel region, we have arch_kexec_protect_crashkres() to secure
the region, and crash_shrink_memory() could be called to shrink it.
While crahshkernel region could be crossig part of a block mapping or section
mapping and the mapping need be splitted, that will cause TLB conflicts.

> 
> I found Mark replay "arm64: remove page granularity limitation from
> KFENCE"[1],
> 
>   "We also avoid live changes from block<->table mappings, since the
>   archtitecture gives us very weak guarantees there and generally requires
>   a Break-Before-Make sequence (though IIRC this was tightened up
>   somewhat, so maybe going one way is supposed to work). Unless it's
>   really necessary, I'd rather not split these block mappings while
>   they're live."
> 
> Hi Mark and Catalin,  could you give some comment,  many thanks.
> 
> [1] https://lore.kernel.org/lkml/20210920101938.GA13863@C02TD0UTHF1T.local/T/#m1a7f974593f5545cbcfc0d21560df4e7926b1381
> 
> 
> > 
> > However, I would suggest letting it as is with below reasons:
> > 
> > 1) The code will complicate the crashkernel reservatoin code which
> > is already difficult to understand.
> > 2) It can only optimize the two cases, first is CONFIG_ZONE_DMA|DMA32
> >    disabled, the other is crashkernel=,high is specified. While both
> >    two cases are corner case, most of systems have CONFIG_ZONE_DMA|DMA32
> >    enabled, and most of systems have crashkernel=xM which is enough.
> >    Having them optimized won't bring benefit to most of systems.
> > 3) Besides, the crashkernel=,high can be handled earlier because
> >    arm64 alwasys have memblock.bottom_up == false currently, thus we
> >    don't need worry arbout the lower limit of crashkernel,high
> >    reservation for now. If memblock.bottom_up is set true in the future,
> >    this patch doesn't work any more.
> > 
> > 
> > ...
> >          crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> >                                                 crash_base, crash_max);
> > 
> > So, in my opinion, we can leave the current NON_BLOCK|SECT mapping as
> > is caused by crashkernel reserving, since no regression is brought.
> > And meantime, turning to check if there's any way to make the contiguous
> > linear mapping and later splitting work. The patch 4, 5 in this patchset
> > doesn't make much sense to me, frankly speaking.
> > 
> > Thanks
> > Baoquan
>