On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote: > 在 2022/7/4 21:15, Will Deacon 写道: > > On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote: > > > > > 1.Quoted messages from arch/arm64/mm/init.c > > > > > > > > > > "Memory reservation for crash kernel either done early or deferred > > > > > depending on DMA memory zones configs (ZONE_DMA) -- > > > > > > > > > > In absence of ZONE_DMA configs arm64_dma_phys_limit initialized > > > > > here instead of max_zone_phys(). This lets early reservation of > > > > > crash kernel memory which has a dependency on arm64_dma_phys_limit. > > > > > Reserving memory early for crash kernel allows linear creation of block > > > > > mappings (greater than page-granularity) for all the memory bank rangs. > > > > > In this scheme a comparatively quicker boot is observed. > > > > > > > > > > If ZONE_DMA configs are defined, crash kernel memory reservation > > > > > is delayed until DMA zone memory range size initialization performed in > > > > > zone_sizes_init(). The defer is necessary to steer clear of DMA zone > > > > > memory range to avoid overlap allocation. > > > > > > > > > > [[[ > > > > > So crash kernel memory boundaries are not known when mapping all bank memory > > > > > ranges, which otherwise means not possible to exclude crash kernel range > > > > > from creating block mappings so page-granularity mappings are created for > > > > > the entire memory range. > > > > > ]]]" > > > > > > > > > > Namely, the init order: memblock init--->linear mem mapping(4k mapping for > > > > > crashkernel, requirinig page-granularity changing))--->zone dma > > > > > limit--->reserve crashkernel. > > > > > So when enable ZONE DMA and using crashkernel, the mem mapping using 4k > > > > > mapping. > > > > > > > > Yes, I understand that is how things work today but I'm saying that we may > > > > as well leave the crashkernel mapped (at block granularity) if > > > > !can_set_direct_map() and then I think your patch becomes a lot simpler. > > > > > > But Page-granularity mapppings are necessary for crash kernel memory range > > > for shrinking its size via /sys/kernel/kexec_crash_size interfac(Quoted from > > > arch/arm64/mm/init.c). > > > So this patch split block/section mapping to 4k page-granularity mapping for > > > crashkernel mem. > > > > Why? I don't see why the mapping granularity is relevant at all if we > > always leave the whole thing mapped. > > > There is another reason. > > When loading crashkernel finish, the do_kexec_load will use > arch_kexec_protect_crashkres to invalid all the pagetable for crashkernel > mem(protect crashkernel mem from access). > > arch_kexec_protect_crashkres--->set_memory_valid--->...--->apply_to_pmd_range > > In the apply_to_pmd_range, there is a judement： BUG_ON(pud_huge(*pud)). And > if the crashkernel use block/section mapping, there will be some error. > > Namely, it's need to use non block/section mapping for crashkernel mem > before shringking. Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do that if we're leaving the thing mapped, no? Will
