Hi Catalin, On 2020/7/29 19:58, Catalin Marinas wrote: > Hi Chen, > > On Wed, Jul 29, 2020 at 11:52:39AM +0800, chenzhou wrote: >> On 2020/7/28 1:30, Catalin Marinas wrote: >>> Anyway, there are two series solving slightly different issues with >>> kdump reservations: >>> >>> 1. This series which relaxes the crashkernel= allocation to go anywhere >>> in the accessible space while having a dedicated crashkernel=X,low >>> option for ZONE_DMA. >>> >>> 2. Bhupesh's series [1] forcing crashkernel=X allocations only from >>> ZONE_DMA. >>> >>> For RPi4 support, we limited ZONE_DMA allocations to the 1st GB. >>> Existing crashkernel= uses may no longer work, depending on where the >>> allocation falls. Option (2) above is a quick fix assuming that the >>> crashkernel reservation is small enough. What's a typical crashkernel >>> option here? That series is probably more prone to reservation failures. >>> >>> Option (1), i.e. this series, doesn't solve the problem raised by >>> Bhupesh unless one uses the crashkernel=X,low argument. It can actually >>> make it worse even for ZONE_DMA32 since the allocation can go above 4G >>> (assuming that we change the ZONE_DMA configuration to only limit it to >>> 1GB on RPi4). >>> >>> I'm more inclined to keep the crashkernel= behaviour to ZONE_DMA >>> allocations. If this is too small for typical kdump, we can look into >>> expanding ZONE_DMA to 4G on non-RPi4 hardware (we had patches on the >>> list). In addition, if Chen thinks allocations above 4G are still needed >>> or if RPi4 needs a sufficiently large crashkernel=, I'd rather have a >>> ",high" option to explicitly require such access. >> Thanks for your reply and exhaustive explanation. >> >> In our ARM servers, we need to to reserve a large chunk for kdump(512M >> or 1G), there is no enough low memory. So we proposed this patch >> series "support reserving crashkernel above 4G on arm64 kdump" In >> April 2019. > Trying to go through the discussions last year, hopefully things get > clearer. > > So prior to the ZONE_DMA change, you still couldn't reserve 1G in the > first 4GB? It shouldn't be sparsely populated during early boot. Yes, we prior to the ZONE_DMA change, you still couldn't reserve 1G/512M in the first 4GB. The memory reported by the bios may be splitted by some "reserved" entries. Like this: ... 2f126000-2fbfffff : reserved 2fc00000-396affff : System RAM 30de8000-30de9fff : reserved 30dec000-30decfff : reserved 30df2000-30df2fff : reserved 30e20000-30e4ffff : reserved 39620000-3968ffff : reserved 396b0000-3974ffff : reserved 39750000-397affff : System RAM 397b0000-398fffff : reserved 39900000-3990ffff : System RAM 39900000-3990ffff : reserved ... > >> I introduce parameters "crashkernel=X,[high,low]" as x86_64 does in earlier versions. >> Suggested by James, to simplify, we call reserve_crashkernel_low() at the beginning of >> reserve_crashkernel() and then relax the arm64_dma32_phys_limit if reserve_crashkernel_low() >> allocated something. >> That is, just the parameter "crashkernel=X,low" is ok and i deleted "crashkernel=X,high". > The problem I see is that with your patches we diverge from x86 > behaviour (and the arm64 behaviour prior to the ZONE_DMA reduction) as > we now require that crashkernel=X,low is always passed if you want > something in ZONE_DMA (and you do want, otherwise the crashdump kernel > fails to boot). > > My main requirement is that crashkernel=X, without any suffix, still > works which I don't think is guaranteed with your patches (well, > ignoring RPi4 ZONE_DMA). Bhupesh's series is a quick fix but doesn't > solve your large allocation requirements (that may have worked prior to > the ZONE_DMA change). The main purpose of this series is to solve the large allocation requirements. Before the DMA_ZONE, both the original crashkernel=X and large allocation with my patches work well. With the DMA_ZONE, both the original crashkernel=X and large allocation with my patches may fail to boot. Both need to think about the DMA_ZONE. > >> After the ZONE_DMA introduced in December 2019, the issue occurred as >> you said above. In fact, we didn't have RPi4 machine. > You don't even need to have a RPi4 machine, ZONE_DMA has been set to 1GB > unconditionally. And while we could move it back to 4GB on non-RPi4 > hardware, I'd rather have a solution that fixes kdump for RPi4 as well. > >> Originally, i suggested to fix this based on this patch series and >> used the dedicated option. >> >> According to your clarify, for typical kdump, there are other >> solutions. In this case, "keep the crashkernel= behaviour to ZONE_DMA >> allocations" looks much better. >> >> How about like this: >> 1. For ZONE_DMA issue, use Bhupesh's solution, keep the crashkernel= >> behaviour to ZONE_DMA allocations. >> 2. For this patch series, make the reserve_crashkernel_low() to >> ZONE_DMA allocations. > So you mean rebasing your series on top of Bhupesh's? I guess you can > combine the two, I really don't care which way as long as we fix both > issues and agree on the crashkernel= semantics. I think with some tweaks > we can go with your series alone. > > IIUC from the x86 code (especially the part you #ifdef'ed out for > arm64), if ",low" is not passed (so just standard crashkernel=X), it > still allocates sufficient low memory for the swiotlb in ZONE_DMA. The > rest can go in a high region. Why can't we do something similar on > arm64? Of course, you can keep the ",low" argument for explicit > allocation but I don't want to mandate it. It is a good idea to combine the two. For parameter crashkernel=X, we do like this: 1. allocate some low memory in ZONE_DMA(or ZONE_DMA32 if CONFIG_ZONE_DMA=n) 2. allocate X size memory in a high region ",low" argument can be used to specify the low memory. Do i understand correctly? > > So with an implicit ZONE_DMA allocation similar to the x86 one, we > probably don't need Bhupesh's series at all. In addition, we can limit > crashkernel= to the first 4G with a fall-back to high like x86 (not sure > if memblock_find_in_range() is guaranteed to search in ascending order). > I don't think we need an explicit ",high" annotation. > > So with the above, just a crashkernel=1G gives you at least 256MB in > ZONE_DMA followed by the rest anywhere, with a preference for > ZONE_DMA32. This way we can also keep the reserve_crashkernel_low() > mostly intact from x86 (less #ifdef's). > > Do I miss anything? Yes. We can let crashkernel=X try to reserve low memory and fall back to use high memory if failing to find a low range. About the function reserve_crashkernel_low(), if we put it in arch/arm64, there is some common code with x86_64. Some suggestions about this? Thanks, Chen Zhou >
