linux-kernel - Re: [PATCH v4] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

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Message-ID: <Z31--x4unDHRU5Zo@arm.com>
Date: Tue, 7 Jan 2025 19:22:35 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Zhenhua Huang <quic_zhenhuah@...cinc.com>
Cc: anshuman.khandual@....com, will@...nel.org, ardb@...nel.org,
	ryan.roberts@....com, mark.rutland@....com, joey.gouly@....com,
	dave.hansen@...ux.intel.com, akpm@...ux-foundation.org,
	chenfeiyang@...ngson.cn, chenhuacai@...nel.org, linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	quic_tingweiz@...cinc.com, stable@...r.kernel.org
Subject: Re: [PATCH v4] arm64: mm: Populate vmemmap/linear at the page level
 for hotplugged sections

On Tue, Jan 07, 2025 at 03:42:52PM +0800, Zhenhua Huang wrote:
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e2739b69e11b..5e0f514de870 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -42,9 +42,11 @@
>  #include <asm/pgalloc.h>
>  #include <asm/kfence.h>
>  
> -#define NO_BLOCK_MAPPINGS	BIT(0)
> -#define NO_CONT_MAPPINGS	BIT(1)
> -#define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
> +#define NO_PMD_BLOCK_MAPPINGS	BIT(0)
> +#define NO_PUD_BLOCK_MAPPINGS	BIT(1)  /* Hotplug case: do not want block mapping for PUD */
> +#define NO_BLOCK_MAPPINGS (NO_PMD_BLOCK_MAPPINGS | NO_PUD_BLOCK_MAPPINGS)

Nit: please use a tab instead of space before (NO_PMD_...)

> +#define NO_CONT_MAPPINGS	BIT(2)
> +#define NO_EXEC_MAPPINGS	BIT(3)	/* assumes FEAT_HPDS is not used */
>  
>  u64 kimage_voffset __ro_after_init;
>  EXPORT_SYMBOL(kimage_voffset);
> @@ -254,7 +256,7 @@ static void init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  
>  		/* try section mapping first */
>  		if (((addr | next | phys) & ~PMD_MASK) == 0 &&
> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> +		    (flags & NO_PMD_BLOCK_MAPPINGS) == 0) {
>  			pmd_set_huge(pmdp, phys, prot);
>  
>  			/*
> @@ -356,10 +358,11 @@ static void alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
>  
>  		/*
>  		 * For 4K granule only, attempt to put down a 1GB block
> +		 * Hotplug case: do not attempt 1GB block
>  		 */

I don't think we need this comment added here. The hotplug case is a
decision of the caller, so better to have the comment there.

>  		if (pud_sect_supported() &&
>  		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> +		   (flags & NO_PUD_BLOCK_MAPPINGS) == 0) {
>  			pud_set_huge(pudp, phys, prot);

Nit: something wrong with the alignment here. I think the unmodified
line after the 'if' one above was misaligned before your patch.

>  
>  			/*
> @@ -1175,9 +1178,21 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
>  int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  		struct vmem_altmap *altmap)
>  {
> +	unsigned long start_pfn;
> +	struct mem_section *ms;
> +
>  	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>  
> -	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> +	start_pfn = page_to_pfn((struct page *)start);
> +	ms = __pfn_to_section(start_pfn);

Hmm, it would have been better if the core code provided the start pfn
as it does for vmemmap_populate_compound_pages() but I'm fine with
deducting it from 'start'.

> +	/*
> +	 * Hotplugged section does not support hugepages as
> +	 * PMD_SIZE (hence PUD_SIZE) section mapping covers
> +	 * struct page range that exceeds a SUBSECTION_SIZE
> +	 * i.e 2MB - for all available base page sizes.
> +	 */
> +	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || !early_section(ms))
>  		return vmemmap_populate_basepages(start, end, node, altmap);
>  	else
>  		return vmemmap_populate_hugepages(start, end, node, altmap);
> @@ -1339,9 +1354,25 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  		    struct mhp_params *params)
>  {
>  	int ret, flags = NO_EXEC_MAPPINGS;
> +	unsigned long start_pfn = page_to_pfn((struct page *)start);
> +	struct mem_section *ms = __pfn_to_section(start_pfn);

This looks wrong. 'start' here is a physical address, you want
PFN_DOWN() instead.

>  
>  	VM_BUG_ON(!mhp_range_allowed(start, size, true));
>  
> +	/* should not be invoked by early section */
> +	WARN_ON(early_section(ms));
> +
> +	/*
> +	 * 4K base page's PMD_SIZE matches SUBSECTION_SIZE i.e 2MB. Hence
> +	 * PMD section mapping can be allowed, but only for 4K base pages.
> +	 * Where as PMD_SIZE (hence PUD_SIZE) for other page sizes exceed
> +	 * SUBSECTION_SIZE.
> +	 */
> +	if (IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> +		flags |= NO_PUD_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

In theory we can allow contiguous PTE mappings but not PMD. You could
probably do the same as a NO_BLOCK_MAPPINGS and split it into multiple
components - NO_PTE_CONT_MAPPINGS and so on.

> +	else
> +		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

Similarly with 16K/64K pages we can allow contiguous PTEs as they all go
up to 2MB blocks.

I think we should write the flags setup in a more readable way than
trying to do mental maths on the possible combinations, something like:

	flags = NO_PUD_BLOCK_MAPPINGS | NO_PMD_CONT_MAPPINGS;
	if (SUBSECTION_SHIFT < PMD_SHIFT)
		flags |= NO_PMD_BLOCK_MAPPINGS;
	if (SUBSECTION_SHIFT < CONT_PTE_SHIFT)
		flags |= NO_PTE_CONT_MAPPINGS;

This way we don't care about the page size and should cover any changes
to SUBSECTION_SHIFT making it smaller than 2MB.

-- 
Catalin