On Sun, Aug 28, 2022 at 04:37:29PM +0800, Baoquan He wrote: > On 08/25/22 at 10:48am, Mike Rapoport wrote: > ...... > > > > 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. > > Sorry, I misunderstood it. I just went through the log and didn't > look into codes. > > But honestly, if we accept the fallback which doesn't do the protection, > we should be able to take off the protection completely, right? > Otherwise, the reservation code is a little complicated. We don't do protection of the crash kernel for most architectures supporting kexec ;-) My goal was to allow large systems with ZONE_DMA/DMA32 have block mappings in the linear map and crash kernel protection without breaking backward compatibility for the existing systems. > > 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. > > I have sent out the patches with an attempt, it's pretty straightforward > and simple. Because arm64 only has one exception, namely Raspberry Pi 4, > on which some peripherals can only address 30bit range. That is a corner > case, to be honest. And kdump is a necessary feature on server, but may > not be so expected on Raspberry Pi 4, a system for computer education > and hobbyists. And kdump only cares whether the dump target devices can > address 32bit range, namely storage device or network card on server. > If finally confirmed that storage devices can only address 30bit range > on Raspberry Pi 4, people still can have crashkernel=xM@yM method to > reserve crashkernel regions. I hope you are right and Raspberry Pi 4 is the only system that limits DMA'able range to 30 bits. But with diversity of arm64 chips and boards I won't be surprised that there are other variants with a similar problem. > Thanks > Baoquan > -- Sincerely yours, Mike.
