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Date:   Fri, 1 Jul 2022 18:24:28 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Guanghui Feng <guanghuifeng@linux.alibaba.com>
Cc:     baolin.wang@linux.alibaba.com, will@kernel.org,
        akpm@linux-foundation.org, david@redhat.com, jianyong.wu@arm.com,
        james.morse@arm.com, quic_qiancai@quicinc.com,
        christophe.leroy@csgroup.eu, jonathan@marek.ca,
        mark.rutland@arm.com, thunder.leizhen@huawei.com,
        anshuman.khandual@arm.com, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, rppt@kernel.org,
        geert+renesas@glider.be, ardb@kernel.org, linux-mm@kvack.org,
        yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com
Subject: Re: [PATCH v3] arm64: mm: fix linear mapping mem access performance
 degradation
Message-ID: <Yr8tzOJi5CGBl767@arm.com>
References: <1656586222-98555-1-git-send-email-guanghuifeng@linux.alibaba.com>
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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.

-- 
Catalin