Re: [PATCH v3] arm64: mm: fix linear mapping mem access performance degradation

["guanghui.fgh" <guanghuifeng@linux.alibaba.com>]

Thanks.

在 2022/7/2 1:24, Catalin Marinas 写道:
> On Thu, Jun 30, 2022 at 06:50:22PM +0800, Guanghui Feng wrote:
>> +static void init_pmd_remap(pud_t *pudp, unsigned long addr, unsigned long end,
>> +			   phys_addr_t phys, pgprot_t prot,
>> +			   phys_addr_t (*pgtable_alloc)(int), int flags)
>> +{
>> +	unsigned long next;
>> +	pmd_t *pmdp;
>> +	phys_addr_t map_offset;
>> +	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);
> 
> This doesn't follow the architecture requirements for "break before
> make" when changing live page tables. While it may work, it risks
> triggering a TLB conflict abort. The correct sequence normally is:
> 
> 	pmd_clear();
> 	flush_tlb_kernel_range();
> 	__pmd_populate();
> 
> However, do we have any guarantees that the kernel doesn't access the
> pmd range being unmapped temporarily? The page table itself might live
> in one of these sections, so set_pmd() etc. can get a translation fault.
> 
Thanks. I'll follow this.
I'll resend the path as soon as possible.