On 03/23/23 at 05:25pm, Catalin Marinas wrote: > On Mon, Mar 20, 2023 at 09:12:08PM +0800, Baoquan He wrote: > > On 03/17/23 at 06:05pm, Catalin Marinas wrote: > > > On Fri, Mar 17, 2023 at 11:09:13PM +0800, Baoquan He wrote: > > > > In fact, what I want to achieve is we set CRASH_ADDR_LOW_MAX to 4G > > > > fixedly on arm64, just like what we do on x86_64. As for RPi4 platform, > > > > we leave it to crashkernel=size@offset syntax. Two reasons for this: > > > > 1) crashkernel is needed on enterprise platform, such as workstation or > > > > server. While RPi4 is obviously not the target. I contacted several RPi4 > > > > players in Redhat and my friends, none of them ever played kdump > > > > testing. If they really have to, crashkernel=size@offset is enough for > > > > them to set. > > > > > > I'd like crashkernel=size (without @offset) on RPi4 to still do the > > > right thing: a low allocation at least as we had until recently (or > > > high+low where high here is maybe above 1GB). IOW, no regression for > > > this crashkernel=size case. We can then change the explicit > > > crashkernel=x,high to mean only above 4GB irrespective of the platform > > > and crashkernel=x,low to be below arm64_dma_phys_limit. > > > > Since crashkernel=,high and crashkernel=size fallback was added in arm64 > > recently, with my understanding, you are suggesting: > > > > on arm64: > > RPi4: > > crashkernel=size > > 0~1G: low memory (no regression introduced) > > And, if not enough low memory, fall back to memory above 1GB (for RPi4; > it would be above 4GB for any other system). > > > crashkernel=size,high > > 0~1G: low memory | 4G~top: high memory > > Yes. > > > Other normal system: > > crashkernel=size|crashkernel=size,high > > 0~4G: low memory | 4G~top: high memory > > Yes. > > IOW, specifying 'high' only forces the high allocation above 4GB instead > of arm64_dma_phys_limit, irrespective of the platform. If no 'high' > specified search_base remains CRASH_ADDR_LOW_MAX (1GB on RPi4, 4GB for > the rest). > > > > > 2) with the fixed CRASH_ADDR_LOW_MAX as 4G, we can easily fix the > > > > problem of base page mapping for the whole linear mapping if crsahkernel= > > > > is set in kernel parameter shown in [1] at bottom. > > > > > > That's a different problem ;). I should re-read that thread, forgot most > > > of the details but I recall one of the counter arguments was that there > > > isn't a strong case to unmap the crashkernel reservation. Now, if we > > > place crashdump kernel image goes in the 'high' reservation, can we not > > > leave the 'low' reservation mapped? We don't really care about it as it > > > wouldn't have any meaningful code/data to be preserved. If the 'high' > > > one goes above 4G always, we don't depend on the arm64_dma_phys_limit. > > > > Yes, this looks ideal. While it only works when crashkernel=,high case and > > it succeeds to reserve a memory region for the specified size of crashkernel > > high memory. At below, we have 4 cases of crashkernel= syntax: > > > > crashkernel=size > > 1)first attempt: low memory under arm64_dma_phys_limit > > 2)fallback: finding memory above 4G > > (2) should be 'finding memory above arm64_dma_phys_limit' to keep the > current behaviour for RPi4. Then for RPi4, with case 2), it will find memory above arm64_dma_phys_limit, namely 1G. Then it will get two memory regions, one could be in [1G, 4G], another is below 4G. I am fine with this, as long as it won't cause confusion that people may think two low memory regions you mentioned earlier. Please help confirm if I understand your suggestioin correctly. I will start making patch with this clarified. Thanks. > > > crashkernel=size,high > > 3)first attempt: finding memory above 4G > > 4)fallback: low memory under arm64_dma_phys_limit > > Yes. > > > case 3) works with your suggestion. However, 1), 2), 4) all need to > > defer to bootmem_init(). With these cases and different handling, > > reserve_crashkernel() could be too complicated. > > Ah, because of the fallback below arm64_dma_phys_limit as in (4), we > still can't move the full crashkernel reservation early. Well, we could > do it in two steps: (a) early attempt at crashkernel reservation above > 4G if 'high' was specified and we avoid mapping it if successful and (b) > do the late crashkernel reservation below arm64_dma_phys_limit and skip > unmapping as being too late. This way most server-like platforms would > get a reservation above 4G, unmapped. Yeah, this covers case 3), while other cases are still in pit. > > > I am wondering if we can cancel the protection of crashkernel memory > > region on arm64 for now. In earlier discussion, people questioned if the > > protection is necessary on arm64. After comparison, I would rather take > > away the protection method of crashkernel region since they try to > > protect in a chance in one million , while the base page mapping for the > > whole linear mapping is mitigating arm64 high end server always. > > This works for me. We can add the protection later for addresses above > 4GB only as mentioned above. Thanks, I have posted a patchset to cancel the protection on crashkernel memory region as per your confirmation here. This can give distros a chance to back port them to fix the performance issue caused by the base page mapping. I personally expect we can hold the crashkernel region unprotected till we have a ideal solution since the code will be elegant with comfortable simplicity. Let's wait and see the code change if people interested want to keep the protection methods. _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
