On 2023/3/24 22:53, Baoquan He wrote: > 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. I saw the patchset. No protection is also a good idea, the code is simplified a lot. > > . > -- Regards, Zhen Lei _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
