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

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Date:   Sat, 2 Jul 2022 18:12:33 +0800
From:   "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     baolin.wang@...ux.alibaba.com, will@...nel.org,
        akpm@...ux-foundation.org, david@...hat.com, jianyong.wu@....com,
        james.morse@....com, quic_qiancai@...cinc.com,
        christophe.leroy@...roup.eu, jonathan@...ek.ca,
        mark.rutland@....com, thunder.leizhen@...wei.com,
        anshuman.khandual@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, rppt@...nel.org,
        geert+renesas@...der.be, ardb@...nel.org, linux-mm@...ck.org,
        yaohongbo@...ux.alibaba.com, alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH v3] arm64: mm: fix linear mapping mem access performance
 degradation

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.