Re: [PATCH RFC v1] arm64: mm: change mem_map to use block/section mapping with crashkernel

[Catalin Marinas]

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