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

[Baoquan He <bhe@redhat.com>]

Hi Catalin,

On 06/21/22 at 07:04pm, Catalin Marinas wrote:
> On Tue, Jun 21, 2022 at 02:24:01PM +0800, Kefeng Wang wrote:
> > On 2022/6/21 13:33, Baoquan He wrote:
> > > 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?
> > 
> > Could the crashkernel reservation (and Kfence pool) be splited dynamically?
> > 
> > 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."
> 
> The problem with splitting is that you can end up with two entries in
> the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
> for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
> abort (but can be worse like loss of coherency).

Thanks for this explanation. Is this a drawback of arm64 design? X86
code do the same thing w/o issue, is there way to overcome this on
arm64 from hardware or software side?

I ever got a arm64 server with huge memory, w or w/o crashkernel setting 
have different bootup time. And the more often TLB miss and flush will
cause performance cost. It is really a pity if we have very powerful
arm64 cpu and system capacity, but bottlenecked by this drawback.

> 
> Prior to FEAT_BBM (added in ARMv8.4), such scenario was not allowed at
> all, the software would have to unmap the range, TLBI, remap. With
> FEAT_BBM (level 2), we can do this without tearing the mapping down but
> we still need to handle the potential TLB conflict abort. The handler
> only needs a TLBI but if it touches the memory range being changed it
> risks faulting again. With vmap stacks and the kernel image mapped in
> the vmalloc space, we have a small window where this could be handled
> but we probably can't go into the C part of the exception handling
> (tracing etc. may access a kmalloc'ed object for example).
> 
> Another option is to do a stop_machine() (if multi-processor at that
> point), disable the MMUs, modify the page tables, re-enable the MMU but
> it's also complicated.
> 
> -- 
> Catalin
> 


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