On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote: > > > 在 2022/7/4 19:14, Will Deacon 写道: > > On Mon, Jul 04, 2022 at 06:58:20PM +0800, guanghui.fgh wrote: > > > > > > > > > 在 2022/7/4 18:35, Will Deacon 写道: > > > > On Sat, Jul 02, 2022 at 11:57:53PM +0800, Guanghui Feng wrote: > > > > > The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone > > > > > (enable crashkernel, disable rodata full, disable kfence), the mem_map will > > > > > use non block/section mapping(for crashkernel requires to shrink the region > > > > > in page granularity). But it will degrade performance when doing larging > > > > > continuous mem access in kernel(memcpy/memmove, etc). > > > > > > > > Hmm. It seems a bit silly to me that we take special care to unmap the > > > > crashkernel from the linear map even when can_set_direct_map() is false, as > > > > we won't be protecting the main kernel at all! > > > > > > > > Why don't we just leave the crashkernel mapped if !can_set_direct_map() > > > > and then this problem just goes away? > > > > > > This question had been asked lask week. > > > > Sorry, I didn't spot that. Please could you link me to the conversation, as > > I'm still unable to find it in my inbox? > > Please access this link: > https://lore.kernel.org/linux-arm-kernel/075b0a8e-cb7e-70f6-b45a-54cd31886794@xxxxxxxxxxxxxxxxx/T/ Sorry, but I read through the thread and I still can't find where the possibility of leaving the crashkernel mapped was discussed. > > > 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. Will
