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? > > > > Will > > 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? > 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. Will
