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

[Kefeng Wang]

On 2022/6/23 18:27, Catalin Marinas wrote:
> On Wed, Jun 22, 2022 at 08:03:21PM +0800, Kefeng Wang wrote:
>> On 2022/6/22 2:04, 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 your explanation，
>>> 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).
>> So if without FEAT_BBM，we can only guarantee BBM sequence via
>> "unmap the range, TLBI, remap" or the following option,
> Yes, that's the break-before-make sequence.
>
>> and with FEAT_BBM (level 2), we could have easy way to avoid TLB
>> conflict for some vmalloc space, but still hard to deal with other
>> scence?
> It's not too hard in theory. Basically there's a small risk of getting a
> TLB conflict abort for the mappings you change without a BBM sequence (I
> think it's nearly non-existed when going from large block to smaller
> pages, though the architecture states that it's still possible). Since
> we only want to do this for the linear map and the kernel and stack are
> in the vmalloc space, we can handle such trap as an safety measure (it
> just needs a TLBI). It may help to tweak a model to force it to generate
> such conflict aborts, otherwise we'd not be able to test the code.
>
> It's possible that such trap is raised at EL2 if a guest caused the
> conflict abort (the architecture left this as IMP DEF). The hypervisors
> may need to be taught to do a TLBI VMALLS12E1 instead of killing the
> guest. I haven't checked what KVM does.
Got it，many thanks.