Re: [PATCH v5] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

Gentle reminder if you happened to miss it :)

On 2025/1/9 17:38, Zhenhua Huang wrote:
> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> to 27, making one section 128M. The related page struct which vmemmap
> points to is 2M then.
> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> vmemmap to populate at the PMD section level which was suitable
> initially since hot plug granule is always one section(128M). However,
> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> existing arm64 assumptions.
> 
> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
> pmd_sect() is true, the entire PMD section is cleared, even if there is
> other effective subsection. For example page_struct_map1 and
> page_strcut_map2 are part of a single PMD entry and they are hot-added
> sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
> the entire PMD entry freeing the struct page map for the whole section,
> even though page_struct_map2 is still active. Similar problem exists
> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
> Tearing down the entire PMD mapping too will leave other subsections
> unmapped in the linear mapping.
> 
> To address the issue, we need to prevent PMD/PUD/CONT mappings for both
> linear and vmemmap for non-boot sections if corresponding size on the
> given base page exceeds SUBSECTION_SIZE(2MB now).
> 
> Cc: stable@vger.kernel.org # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> ---
> Hi Catalin and Anshuman,
> I have addressed comments so far, please help review.
> One outstanding point which not finalized is in vmemmap_populate(): how to judge hotplug
> section. Currently I am using system_state, discussion:
> https://lore.kernel.org/linux-mm/1515dae4-cb53-4645-8c72-d33b27ede7eb@quicinc.com/
>   arch/arm64/mm/mmu.c | 46 ++++++++++++++++++++++++++++++++++++---------
>   1 file changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e2739b69e11b..8718d6e454c5 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -42,9 +42,13 @@
>   #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)
> +#define NO_PTE_CONT_MAPPINGS	BIT(2)
> +#define NO_PMD_CONT_MAPPINGS	BIT(3)  /* Hotplug case: do not want cont mapping for PMD */
> +#define NO_CONT_MAPPINGS	(NO_PTE_CONT_MAPPINGS | NO_PMD_CONT_MAPPINGS)
> +#define NO_EXEC_MAPPINGS	BIT(4)	/* assumes FEAT_HPDS is not used */
>   
>   u64 kimage_voffset __ro_after_init;
>   EXPORT_SYMBOL(kimage_voffset);
> @@ -224,7 +228,7 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>   
>   		/* use a contiguous mapping if the range is suitably aligned */
>   		if ((((addr | next | phys) & ~CONT_PTE_MASK) == 0) &&
> -		    (flags & NO_CONT_MAPPINGS) == 0)
> +		    (flags & NO_PTE_CONT_MAPPINGS) == 0)
>   			__prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>   
>   		init_pte(ptep, addr, next, phys, __prot);
> @@ -254,7 +258,7 @@ 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);
>   
>   			/*
> @@ -311,7 +315,7 @@ static void alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
>   
>   		/* use a contiguous mapping if the range is suitably aligned */
>   		if ((((addr | next | phys) & ~CONT_PMD_MASK) == 0) &&
> -		    (flags & NO_CONT_MAPPINGS) == 0)
> +		    (flags & NO_PMD_CONT_MAPPINGS) == 0)
>   			__prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>   
>   		init_pmd(pmdp, addr, next, phys, __prot, pgtable_alloc, flags);
> @@ -358,8 +362,8 @@ 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
>   		 */
>   		if (pud_sect_supported() &&
> -		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> +		    ((addr | next | phys) & ~PUD_MASK) == 0 &&
> +		    (flags & NO_PUD_BLOCK_MAPPINGS) == 0) {
>   			pud_set_huge(pudp, phys, prot);
>   
>   			/*
> @@ -1177,7 +1181,13 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>   {
>   	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>   
> -	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
> +	/*
> +	 * 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) || system_state != SYSTEM_BOOTING)
>   		return vmemmap_populate_basepages(start, end, node, altmap);
>   	else
>   		return vmemmap_populate_hugepages(start, end, node, altmap);
> @@ -1339,9 +1349,27 @@ int arch_add_memory(int nid, u64 start, u64 size,
>   		    struct mhp_params *params)
>   {
>   	int ret, flags = NO_EXEC_MAPPINGS;
> +	unsigned long start_pfn = PFN_DOWN(start);
> +	struct mem_section *ms = __pfn_to_section(start_pfn);
>   
>   	VM_BUG_ON(!mhp_range_allowed(start, size, true));
>   
> +	/* should not be invoked by early section */
> +	WARN_ON(early_section(ms));
> +
> +	/*
> +	 * Disallow BlOCK/CONT mappings if the corresponding size exceeds
> +	 * SUBSECTION_SIZE which now is 2MB.
> +	 *
> +	 * PUD_BLOCK or PMD_CONT should consistently exceed SUBSECTION_SIZE
> +	 * across all variable page size configurations, so add them directly
> +	 */
> +	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;
> +
>   	if (can_set_direct_map())
>   		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>