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

[Catalin Marinas]

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).

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