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Message-ID: <9ae36424-2cb6-491d-8ac2-95bfe39828a2@quicinc.com>
Date: Wed, 15 Jan 2025 10:13:21 +0800
From: Zhenhua Huang <quic_zhenhuah@...cinc.com>
To: <anshuman.khandual@....com>, <catalin.marinas@....com>
CC: <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 v5] arm64: mm: Populate vmemmap/linear at the page level
for hotplugged sections
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@...r.kernel.org # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.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;
>
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