Messages in this thread | | | Date | Mon, 4 Jul 2022 22:11:27 +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 21:15, Will Deacon 写道: > 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@linux.alibaba.com/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
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.
Thanks.
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