On 2023/3/24 1:25, 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. > >> 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. > >> 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. Recently, I've also been rethinking the performance issues when kdump is enabled. I have a new idea. For crashkernel=X, we can temporarily search for free memory from the low address to the high address. As below: save_bottom_up = memblock_bottom_up(); if (!high) memblock_set_bottom_up(true); crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN, crash_base, crash_max); memblock_set_bottom_up(save_bottom_up); The final code change should be small, and I'll try it today. > -- Regards, Zhen Lei _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
