Hi Leizhen, On 03/24/23 at 10:47am, Leizhen (ThunderTown) wrote: ...... > >>>> 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. I have sent a patchset to remove the crashkernel region protection code as per Catalin's confirmation. I personally like the code conciseness w/o protection because kinds of crahskernel reservation has been complex, the situation on arm64 will makes it worse if we try to keep the protection and fix the performance issue. While I am glad to see any attempt to achieve the two goals if it's satisfactory. _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
