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Message-ID: <10a970c0-e8f2-41c5-a68f-3ac4ae88d9d0@quicinc.com>
Date: Thu, 6 Feb 2025 16:48:35 +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
Dear Catalin and Anshuman,
Given that it's a genuine bug affecting stability, we need to address it
:). Could you please let me know if you have any additional concerns or
thoughts?
On 2025/1/15 10:13, Zhenhua Huang wrote:
> 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@...cinc.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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