Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled

[Mike Rapoport]

Hi Baoquan,

On Thu, Aug 25, 2022 at 03:35:04PM +0800, Baoquan He wrote:
> Add kexec list in CC
> 
> On 08/19/22 at 07:11am, Mike Rapoport wrote:
> > From: Mike Rapoport <mike.rapoport@xxxxxxxxx>
> > 
> > Hi,
> > 
> > There were several rounds of discussion how to remap with base pages only
> > the crash kernel area, the latest one here:
> > 
> > https://lore.kernel.org/all/1656777473-73887-1-git-send-email-guanghuifeng@xxxxxxxxxxxxxxxxx
> > 
> > and this is my attempt to allow having both large pages in the linear map
> > and protection for the crash kernel memory.
> > 
> > For server systems it is important to protect crash kernel memory for
> > post-mortem analysis, and for that protection to work the crash kernel
> > memory should be mapped with base pages in the linear map. 
> > 
> > On the systems with ZONE_DMA/DMA32 enabled, crash kernel reservation
> > happens after the linear map is created and the current code forces using
> > base pages for the entire linear map, which results in performance
> > degradation.
> > 
> > These patches enable remapping of the crash kernel area with base pages
> > while keeping large pages in the rest of the linear map.
> > 
> > The idea is to align crash kernel reservation to PUD boundaries, remap that
> > PUD and then free the extra memory.
> 
> Hi Mike,
> 
> Thanks for the effort to work on this issue. While I have to say this
> isnt's good because it can only be made relying on a prerequisite that
> there's big enough memory. If on a system, say 2G memory, it's not easy
> to succeed on getting one 1G memory. While we only require far smaller
> region than 1G, e.g about 200M which should be easy to get. So the way
> taken in this patchset is too quirky and will cause regression on
> systemswith small memory. This kind of sytems with small memory exists
> widely on virt guest instance.

I don't agree there is a regression. If the PUD-aligned allocation fails,
there is a fallback to the allocation of the exact size requested for crash
kernel. This allocation just won't get protected.

Also please note, that the changes are only for the case when user didn't
force base-size pages in the linear map, so anything that works now will
work the same way with this set applied.
 
> The crashkernel reservation happens after linear map because the
> reservation needs to know the dma zone boundary, arm64_dma_phys_limit.
> If we can deduce that before bootmem_init(), the reservation can be
> done before linear map. I will make an attempt on that. If still can't
> be accepted, we would like to take off the crashkernel region protection
> on arm64 for now.

I doubt it would be easy because arm64_dma_phys_limit is determined after
parsing of the device tree and there might be memory allocations of
possibly unmapped memory during the parsing.
 
> Thanks
> Baoquan
> 

-- 
Sincerely yours,
Mike.