Messages in this thread | | | Date | Wed, 13 Apr 2022 17:53:38 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH RFC v1] arm64: mm: change mem_map to use block/section mapping with crashkernel |
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On Tue, Apr 12, 2022 at 05:07:56PM +0800, Guanghui Feng wrote: > There are many changes and discussions: > commit 031495635b46 > commit 1a8e1cef7603 > commit 8424ecdde7df > commit 0a30c53573b0 > commit 2687275a5843 > > When using DMA/DMA32 zone and crashkernel, disable rodata full and kfence, > 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). > > This patch firstly do block/section mapping at mem_map, reserve crashkernel > memory. And then walking pagetable to split block/section mapping > to non block/section mapping [only] for crashkernel mem. We will accelerate > mem access about 10-20% performance improvement, and reduce the cpu dTLB miss > conspicuously on some platform with this optimization.
Do you actually have some real world use-cases where this improvement matters? I don't deny that large memcpy over the kernel linear map may be slightly faster but where does this really matter?
> +static void init_crashkernel_pmd(pud_t *pudp, unsigned long addr, > + unsigned long end, phys_addr_t phys, > + pgprot_t prot, > + phys_addr_t (*pgtable_alloc)(int), int flags) > +{ > + phys_addr_t map_offset; > + unsigned long next; > + pmd_t *pmdp; > + pmdval_t pmdval; > + > + pmdp = pmd_offset(pudp, addr); > + do { > + next = pmd_addr_end(addr, end); > + if (!pmd_none(*pmdp) && pmd_sect(*pmdp)) { > + phys_addr_t pte_phys = pgtable_alloc(PAGE_SHIFT); > + pmd_clear(pmdp); > + pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN; > + if (flags & NO_EXEC_MAPPINGS) > + pmdval |= PMD_TABLE_PXN; > + __pmd_populate(pmdp, pte_phys, pmdval); > + flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
The architecture requires us to do a break-before-make here, so pmd_clear(), TLBI, __pmd_populate() - in this order. And that's where it gets tricky, if the kernel happens to access this pmd range while it is unmapped, you'd get a translation fault.
-- Catalin
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