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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
Date: Fri, 1 Jul 2022 18:24:28 +0100	[thread overview]
Message-ID: <Yr8tzOJi5CGBl767@arm.com> (raw)
In-Reply-To: <1656586222-98555-1-git-send-email-guanghuifeng@linux.alibaba.com>

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

WARNING: multiple messages have this Message-ID (diff)
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
Date: Fri, 1 Jul 2022 18:24:28 +0100	[thread overview]
Message-ID: <Yr8tzOJi5CGBl767@arm.com> (raw)
In-Reply-To: <1656586222-98555-1-git-send-email-guanghuifeng@linux.alibaba.com>

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


  parent reply	other threads:[~2022-07-01 17:25 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-30 10:50 [PATCH v3] arm64: mm: fix linear mapping mem access performance degradation Guanghui Feng
2022-06-30 10:50 ` Guanghui Feng
2022-06-30 11:47 ` Kefeng Wang
2022-06-30 11:47   ` Kefeng Wang
2022-06-30 12:53 ` guanghui.fgh
2022-06-30 12:53   ` guanghui.fgh
2022-06-30 13:46 ` Mike Rapoport
2022-06-30 13:46   ` Mike Rapoport
2022-07-01  4:36   ` guanghui.fgh
2022-07-01  4:36     ` guanghui.fgh
2022-07-01 16:51     ` Mike Rapoport
2022-07-01 16:51       ` Mike Rapoport
2022-07-01 17:24 ` Catalin Marinas [this message]
2022-07-01 17:24   ` Catalin Marinas
2022-07-02 10:12   ` guanghui.fgh
2022-07-02 10:12     ` guanghui.fgh
2022-07-02 10:48   ` guanghui.fgh
2022-07-02 10:48     ` guanghui.fgh
2022-07-08 12:13   ` guanghui.fgh
2022-07-08 12:13     ` guanghui.fgh
2022-07-08 14:00     ` Robin Murphy
2022-07-08 14:00       ` Robin Murphy

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