Messages in this thread | | | Date | Mon, 4 Jul 2022 20:05:59 +0800 | Subject | Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation | From | "guanghui.fgh" <> |
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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? >>> >>> 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?
Please access this link: https://lore.kernel.org/linux-arm-kernel/075b0a8e-cb7e-70f6-b45a-54cd31886794@linux.alibaba.com/T/
> >> 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
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.
Thanks.
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