Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled
On 08/29/22 at 05:31pm, Mike Rapoport wrote: > 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-guanghuif...@linux.alibaba.com > > > > > > > > > > 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 ;-) Yeah. The protection was introduced into x86 firstly by my former colleague of Redhat as an enhancement. Later people ported it to arm64. We have signature verification mechanism to check if corruption on loaded kdump kernel happened. In fact, panic is a small probability event, and accidental corruption on kdump kernel data is a much smaller probability event. The protection is an icing on the cake. But if it brings mess, better take it away if no way to clean up the mess. > > 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
Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled
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-guanghuif...@linux.alibaba.com > > > > > > > > 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. ___ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec
Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled
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-guanghuif...@linux.alibaba.com > > > > > > 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. > > 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. Thanks Baoquan ___ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec
Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled
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 > > > > 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-guanghuif...@linux.alibaba.com > > > > 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. ___ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec
Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled
Add kexec list in CC On 08/19/22 at 07:11am, Mike Rapoport wrote: > From: Mike Rapoport > > 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-guanghuif...@linux.alibaba.com > > 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. 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. Thanks Baoquan ___ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec
