[PATCH V2 0/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

[PATCH V2 0/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

[Anshuman Khandual]

This series enables batched TLB flush in unmap_hotplug_range() which avoids
individual page TLB flush for potential CONT blocks in linear mapping while
also improving performance due to range based TLB operation along with less
synchronization barrier instructions.

It also now rejects memory removal that might split a leaf entry in kernel
mapping, which would have otherwise required re-structuring using the break
before make (BBM) semantics. But kernel cannot tolerate BBM, so remapping to
fine grained leaves would not be possible on systems without BBML2_NOABORT.

This series applies on 6.19-rc8 and tested on KVM guest.

Changes in V2:

- Renamed split_kernel_leaf_boundary() as addr_splits_kernel_leaf()
- Dropped pte_valid()/pmd_leaf() before checking pte_cont()/pmd_cont()
- Changed and dropped variables in can_unmap_without_split()
- Moved can_unmap_without_split() after boot memory check
- Renamed 'prevent_bootmem' as 'prevent_memory' across the board
- Updated commit message in both patches

Changes in V1:

- https://lore.kernel.org/all/20260202042617.504183-1-anshuman.khandual@arm.com/

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Anshuman Khandual (2):
  arm64/mm: Enable batched TLB flush in unmap_hotplug_range()
  arm64/mm: Reject memory removal that splits a kernel leaf mapping

 arch/arm64/mm/mmu.c | 236 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 216 insertions(+), 20 deletions(-)

-- 
2.30.2

[PATCH V2 1/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range()

[Anshuman Khandual]

During a memory hot remove operartion both linear and vmemmap mappings for
the memory range being removed, get unmapped via unmap_hotplug_range() but
mapped pages get freed only for vmemmap mapping. This is just a sequential
operation where each table entry gets cleared, followed by a leaf specific
TLB flush, and then followed by memory free operation when applicable.

This approach was simple and uniform both for vmemmap and linear mappings.
But linear mapping might contain CONT marked block memory where it becomes
necessary to first clear out all entire in the range before a TLB flush.
This is as per the architecture requirement. Hence batch all TLB flushes
during the table tear down walk and finally do it in unmap_hotplug_range().

Prior to this fix, it was hypothetically possible for a speculative access
to a higher address in the contiguous block to fill the TLB with shattered
entries for the entire contiguous range after a lower address had already
been cleared and invalidated. Due to the table entries being shattered, the
subsequent TLB invalidation for the higher address would not then clear the
TLB entries for the lower address, meaning stale TLB entries could persist.

Besides it also helps in improving the performance via TLBI range operation
along with reduced synchronization instructions. The time spent executing
unmap_hotplug_range() improved 97% measured over a 2GB memory hot removal
in KVM guest.

This scheme is not applicable during vmemmap mapping tear down where memory
needs to be freed and hence a TLB flush is required after clearing out page
table entry.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
Cc: stable@vger.kernel.org
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/mmu.c | 81 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 67 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 8e1d80a7033e..8ec8a287aaa1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1458,10 +1458,32 @@ static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
 
 		WARN_ON(!pte_present(pte));
 		__pte_clear(&init_mm, addr, ptep);
-		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-		if (free_mapped)
+		if (free_mapped) {
+			/*
+			 * If page is part of an existing contiguous
+			 * memory block, individual TLB invalidation
+			 * here would not be appropriate. Instead it
+			 * will require clearing all entries for the
+			 * memory block and subsequently a TLB flush
+			 * for the entire range.
+			 */
+			WARN_ON(pte_cont(pte));
+
+			/*
+			 * TLB flush is essential for freeing memory.
+			 */
+			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 			free_hotplug_page_range(pte_page(pte),
 						PAGE_SIZE, altmap);
+		}
+
+		/*
+		 * TLB flush is batched in unmap_hotplug_range()
+		 * for the entire range, when memory need not be
+		 * freed. Besides linear mapping might have CONT
+		 * blocks where TLB flush needs to be done after
+		 * clearing all relevant entries.
+		 */
 	} while (addr += PAGE_SIZE, addr < end);
 }
 
@@ -1482,15 +1504,32 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
 		WARN_ON(!pmd_present(pmd));
 		if (pmd_sect(pmd)) {
 			pmd_clear(pmdp);
+			if (free_mapped) {
+				/*
+				 * If page is part of an existing contiguous
+				 * memory block, individual TLB invalidation
+				 * here would not be appropriate. Instead it
+				 * will require clearing all entries for the
+				 * memory block and subsequently a TLB flush
+				 * for the entire range.
+				 */
+				WARN_ON(pmd_cont(pmd));
+
+				/*
+				 * TLB flush is essential for freeing memory.
+				 */
+				flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+				free_hotplug_page_range(pmd_page(pmd),
+							PMD_SIZE, altmap);
+			}
 
 			/*
-			 * One TLBI should be sufficient here as the PMD_SIZE
-			 * range is mapped with a single block entry.
+			 * TLB flush is batched in unmap_hotplug_range()
+			 * for the entire range, when memory need not be
+			 * freed. Besides linear mapping might have CONT
+			 * blocks where TLB flush needs to be done after
+			 * clearing all relevant entries.
 			 */
-			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-			if (free_mapped)
-				free_hotplug_page_range(pmd_page(pmd),
-							PMD_SIZE, altmap);
 			continue;
 		}
 		WARN_ON(!pmd_table(pmd));
@@ -1515,15 +1554,20 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
 		WARN_ON(!pud_present(pud));
 		if (pud_sect(pud)) {
 			pud_clear(pudp);
+			if (free_mapped) {
+				/*
+				 * TLB flush is essential for freeing memory.
+				 */
+				flush_tlb_kernel_range(addr, addr + PUD_SIZE);
+				free_hotplug_page_range(pud_page(pud),
+							PUD_SIZE, altmap);
+			}
 
 			/*
-			 * One TLBI should be sufficient here as the PUD_SIZE
-			 * range is mapped with a single block entry.
+			 * TLB flush is batched in unmap_hotplug_range()
+			 * for the entire range, when memory need not be
+			 * freed.
 			 */
-			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-			if (free_mapped)
-				free_hotplug_page_range(pud_page(pud),
-							PUD_SIZE, altmap);
 			continue;
 		}
 		WARN_ON(!pud_table(pud));
@@ -1553,6 +1597,7 @@ static void unmap_hotplug_p4d_range(pgd_t *pgdp, unsigned long addr,
 static void unmap_hotplug_range(unsigned long addr, unsigned long end,
 				bool free_mapped, struct vmem_altmap *altmap)
 {
+	unsigned long start = addr;
 	unsigned long next;
 	pgd_t *pgdp, pgd;
 
@@ -1574,6 +1619,14 @@ static void unmap_hotplug_range(unsigned long addr, unsigned long end,
 		WARN_ON(!pgd_present(pgd));
 		unmap_hotplug_p4d_range(pgdp, addr, next, free_mapped, altmap);
 	} while (addr = next, addr < end);
+
+	/*
+	 * Batched TLB flush only for linear mapping which
+	 * might contain CONT blocks, and does not require
+	 * freeing up memory as well.
+	 */
+	if (!free_mapped)
+		flush_tlb_kernel_range(start, end);
 }
 
 static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
-- 
2.30.2

[PATCH V2 2/2] arm64/mm: Reject memory removal that splits a kernel leaf mapping

[Anshuman Khandual]

Linear and vmemmap mapings that get teared down during a memory hot remove
operation might contain leaf level entries on any page table level. If the
requested memory range's linear or vmemmap mappings falls within such leaf
entries, new mappings need to be created for the remaning memory mapped on
the leaf entry earlier, following standard break before make aka BBM rules.
But kernel cannot tolerate BBM amd hemce remapping to fine grained leaves
would not be possible on systems without BBML2_NOABORT.

Currently memory hot remove operation does not perform such restructuring,
and so removing memory ranges that could split a kernel leaf level mapping
need to be rejected.

while memory_hotplug.c does appear to permit hot removing arbitrary ranges
of memory, the higher layers that drive memory_hotplug (e.g. ACPI, virtio,
...) all appear to treat memory as fixed size devices. So it is impossible
to hotunplug a different amount than was previously hotplugged, and hence
we should never see a rejection in practice, but adding the check makes us
robust against a future change.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
Cc: stable@vger.kernel.org
Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/mmu.c | 155 ++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 149 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 8ec8a287aaa1..3fb9bcbd739a 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -2063,6 +2063,142 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
 	__remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size);
 }
 
+
+static bool addr_splits_kernel_leaf(unsigned long addr)
+{
+	pgd_t *pgdp, pgd;
+	p4d_t *p4dp, p4d;
+	pud_t *pudp, pud;
+	pmd_t *pmdp, pmd;
+	pte_t *ptep, pte;
+
+	/*
+	 * PGD level:
+	 *
+	 * If addr is PGD_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, PGDIR_SIZE) == addr)
+		return false;
+
+	pgdp = pgd_offset_k(addr);
+	pgd = pgdp_get(pgdp);
+	if (!pgd_present(pgd))
+		return false;
+
+	/*
+	 * P4D level:
+	 *
+	 * If addr is P4D_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, P4D_SIZE) == addr)
+		return false;
+
+	p4dp = p4d_offset(pgdp, addr);
+	p4d = p4dp_get(p4dp);
+	if (!p4d_present(p4d))
+		return false;
+
+	/*
+	 * PUD level:
+	 *
+	 * If addr is PUD_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, PUD_SIZE) == addr)
+		return false;
+
+	pudp = pud_offset(p4dp, addr);
+	pud = pudp_get(pudp);
+	if (!pud_present(pud))
+		return false;
+
+	if (pud_leaf(pud))
+		return true;
+
+	/*
+	 * CONT_PMD level:
+	 *
+	 * If addr is CONT_PMD_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, CONT_PMD_SIZE) == addr)
+		return false;
+
+	pmdp = pmd_offset(pudp, addr);
+	pmd = pmdp_get(pmdp);
+	if (!pmd_present(pmd))
+		return false;
+
+	if (pmd_cont(pmd))
+		return true;
+
+	/*
+	 * PMD level:
+	 *
+	 * If addr is PMD_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
+		return false;
+
+	if (pmd_leaf(pmd))
+		return true;
+
+	/*
+	 * CONT_PTE level:
+	 *
+	 * If addr is CONT_PTE_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, CONT_PTE_SIZE) == addr)
+		return false;
+
+	ptep = pte_offset_kernel(pmdp, addr);
+	pte = __ptep_get(ptep);
+	if (!pte_present(pte))
+		return false;
+
+	if (pte_cont(pte))
+		return true;
+
+	/*
+	 * PTE level:
+	 *
+	 * If addr is PAGE_SIZE aligned - already on a leaf boundary
+	 */
+	if (ALIGN_DOWN(addr, PAGE_SIZE) == addr)
+		return false;
+	return true;
+}
+
+static bool can_unmap_without_split(unsigned long pfn, unsigned long nr_pages)
+{
+	unsigned long phys_start, phys_end, size, start, end;
+
+	phys_start = PFN_PHYS(pfn);
+	phys_end = phys_start + nr_pages * PAGE_SIZE;
+
+	/*
+	 * PFN range's linear map edges are leaf entry aligned
+	 */
+	start = __phys_to_virt(phys_start);
+	end =  __phys_to_virt(phys_end);
+	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
+		pr_warn("[%lx %lx] splits a leaf entry in linear map\n",
+			phys_start, phys_end);
+		return false;
+	}
+
+	/*
+	 * PFN range's vmemmap edges are leaf entry aligned
+	 */
+	size = nr_pages * sizeof(struct page);
+	start = (unsigned long)pfn_to_page(pfn);
+	end = start + size;
+	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
+		pr_warn("[%lx %lx] splits a leaf entry in vmemmap\n",
+			phys_start, phys_end);
+		return false;
+	}
+	return true;
+}
+
 /*
  * This memory hotplug notifier helps prevent boot memory from being
  * inadvertently removed as it blocks pfn range offlining process in
@@ -2071,8 +2207,11 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
  * In future if and when boot memory could be removed, this notifier
  * should be dropped and free_hotplug_page_range() should handle any
  * reserved pages allocated during boot.
+ *
+ * This also blocks any memory remove that would have caused a split
+ * in leaf entry in kernel linear or vmemmap mapping.
  */
-static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
+static int prevent_memory_remove_notifier(struct notifier_block *nb,
 					   unsigned long action, void *data)
 {
 	struct mem_section *ms;
@@ -2118,11 +2257,15 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
 			return NOTIFY_DONE;
 		}
 	}
+
+	if (!can_unmap_without_split(pfn, arg->nr_pages))
+		return NOTIFY_BAD;
+
 	return NOTIFY_OK;
 }
 
-static struct notifier_block prevent_bootmem_remove_nb = {
-	.notifier_call = prevent_bootmem_remove_notifier,
+static struct notifier_block prevent_memory_remove_nb = {
+	.notifier_call = prevent_memory_remove_notifier,
 };
 
 /*
@@ -2172,7 +2315,7 @@ static void validate_bootmem_online(void)
 	}
 }
 
-static int __init prevent_bootmem_remove_init(void)
+static int __init prevent_memory_remove_init(void)
 {
 	int ret = 0;
 
@@ -2180,13 +2323,13 @@ static int __init prevent_bootmem_remove_init(void)
 		return ret;
 
 	validate_bootmem_online();
-	ret = register_memory_notifier(&prevent_bootmem_remove_nb);
+	ret = register_memory_notifier(&prevent_memory_remove_nb);
 	if (ret)
 		pr_err("%s: Notifier registration failed %d\n", __func__, ret);
 
 	return ret;
 }
-early_initcall(prevent_bootmem_remove_init);
+early_initcall(prevent_memory_remove_init);
 #endif
 
 pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
-- 
2.30.2

Re: [PATCH V2 2/2] arm64/mm: Reject memory removal that splits a kernel leaf mapping

[Ryan Roberts]

On 03/02/2026 13:03, Anshuman Khandual wrote:
> Linear and vmemmap mapings that get teared down during a memory hot remove
> operation might contain leaf level entries on any page table level. If the
> requested memory range's linear or vmemmap mappings falls within such leaf
> entries, new mappings need to be created for the remaning memory mapped on
> the leaf entry earlier, following standard break before make aka BBM rules.
> But kernel cannot tolerate BBM amd hemce remapping to fine grained leaves

nits:                            ^^^ ^^^^^

> would not be possible on systems without BBML2_NOABORT.
> 
> Currently memory hot remove operation does not perform such restructuring,
> and so removing memory ranges that could split a kernel leaf level mapping
> need to be rejected.
> 
> while memory_hotplug.c does appear to permit hot removing arbitrary ranges
> of memory, the higher layers that drive memory_hotplug (e.g. ACPI, virtio,
> ...) all appear to treat memory as fixed size devices. So it is impossible
> to hotunplug a different amount than was previously hotplugged, and hence
> we should never see a rejection in practice, but adding the check makes us
> robust against a future change.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
> Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
> Cc: stable@vger.kernel.org

Given your statement in the commit log above about how this never happens in
practice but is instead to make us robust to any future changes, perhaps this is
not a stable candidate?

For clarity; I do agree that this extra robustness is useful so I think we
should aim to get it upstream, I just don't think it needs to go to stable.

> Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Other than above nit, LGTM:

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

> ---
>  arch/arm64/mm/mmu.c | 155 ++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 149 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 8ec8a287aaa1..3fb9bcbd739a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -2063,6 +2063,142 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
>  	__remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size);
>  }
>  
> +
> +static bool addr_splits_kernel_leaf(unsigned long addr)
> +{
> +	pgd_t *pgdp, pgd;
> +	p4d_t *p4dp, p4d;
> +	pud_t *pudp, pud;
> +	pmd_t *pmdp, pmd;
> +	pte_t *ptep, pte;
> +
> +	/*
> +	 * PGD level:
> +	 *
> +	 * If addr is PGD_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, PGDIR_SIZE) == addr)
> +		return false;
> +
> +	pgdp = pgd_offset_k(addr);
> +	pgd = pgdp_get(pgdp);
> +	if (!pgd_present(pgd))
> +		return false;
> +
> +	/*
> +	 * P4D level:
> +	 *
> +	 * If addr is P4D_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, P4D_SIZE) == addr)
> +		return false;
> +
> +	p4dp = p4d_offset(pgdp, addr);
> +	p4d = p4dp_get(p4dp);
> +	if (!p4d_present(p4d))
> +		return false;
> +
> +	/*
> +	 * PUD level:
> +	 *
> +	 * If addr is PUD_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, PUD_SIZE) == addr)
> +		return false;
> +
> +	pudp = pud_offset(p4dp, addr);
> +	pud = pudp_get(pudp);
> +	if (!pud_present(pud))
> +		return false;
> +
> +	if (pud_leaf(pud))
> +		return true;
> +
> +	/*
> +	 * CONT_PMD level:
> +	 *
> +	 * If addr is CONT_PMD_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, CONT_PMD_SIZE) == addr)
> +		return false;
> +
> +	pmdp = pmd_offset(pudp, addr);
> +	pmd = pmdp_get(pmdp);
> +	if (!pmd_present(pmd))
> +		return false;
> +
> +	if (pmd_cont(pmd))
> +		return true;
> +
> +	/*
> +	 * PMD level:
> +	 *
> +	 * If addr is PMD_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
> +		return false;
> +
> +	if (pmd_leaf(pmd))
> +		return true;
> +
> +	/*
> +	 * CONT_PTE level:
> +	 *
> +	 * If addr is CONT_PTE_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, CONT_PTE_SIZE) == addr)
> +		return false;
> +
> +	ptep = pte_offset_kernel(pmdp, addr);
> +	pte = __ptep_get(ptep);
> +	if (!pte_present(pte))
> +		return false;
> +
> +	if (pte_cont(pte))
> +		return true;
> +
> +	/*
> +	 * PTE level:
> +	 *
> +	 * If addr is PAGE_SIZE aligned - already on a leaf boundary
> +	 */
> +	if (ALIGN_DOWN(addr, PAGE_SIZE) == addr)
> +		return false;
> +	return true;
> +}
> +
> +static bool can_unmap_without_split(unsigned long pfn, unsigned long nr_pages)
> +{
> +	unsigned long phys_start, phys_end, size, start, end;
> +
> +	phys_start = PFN_PHYS(pfn);
> +	phys_end = phys_start + nr_pages * PAGE_SIZE;
> +
> +	/*
> +	 * PFN range's linear map edges are leaf entry aligned
> +	 */
> +	start = __phys_to_virt(phys_start);
> +	end =  __phys_to_virt(phys_end);
> +	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
> +		pr_warn("[%lx %lx] splits a leaf entry in linear map\n",
> +			phys_start, phys_end);
> +		return false;
> +	}
> +
> +	/*
> +	 * PFN range's vmemmap edges are leaf entry aligned
> +	 */
> +	size = nr_pages * sizeof(struct page);
> +	start = (unsigned long)pfn_to_page(pfn);
> +	end = start + size;
> +	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
> +		pr_warn("[%lx %lx] splits a leaf entry in vmemmap\n",
> +			phys_start, phys_end);
> +		return false;
> +	}
> +	return true;
> +}
> +
>  /*
>   * This memory hotplug notifier helps prevent boot memory from being
>   * inadvertently removed as it blocks pfn range offlining process in
> @@ -2071,8 +2207,11 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
>   * In future if and when boot memory could be removed, this notifier
>   * should be dropped and free_hotplug_page_range() should handle any
>   * reserved pages allocated during boot.
> + *
> + * This also blocks any memory remove that would have caused a split
> + * in leaf entry in kernel linear or vmemmap mapping.
>   */
> -static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
> +static int prevent_memory_remove_notifier(struct notifier_block *nb,
>  					   unsigned long action, void *data)
>  {
>  	struct mem_section *ms;
> @@ -2118,11 +2257,15 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
>  			return NOTIFY_DONE;
>  		}
>  	}
> +
> +	if (!can_unmap_without_split(pfn, arg->nr_pages))
> +		return NOTIFY_BAD;
> +
>  	return NOTIFY_OK;
>  }
>  
> -static struct notifier_block prevent_bootmem_remove_nb = {
> -	.notifier_call = prevent_bootmem_remove_notifier,
> +static struct notifier_block prevent_memory_remove_nb = {
> +	.notifier_call = prevent_memory_remove_notifier,
>  };
>  
>  /*
> @@ -2172,7 +2315,7 @@ static void validate_bootmem_online(void)
>  	}
>  }
>  
> -static int __init prevent_bootmem_remove_init(void)
> +static int __init prevent_memory_remove_init(void)
>  {
>  	int ret = 0;
>  
> @@ -2180,13 +2323,13 @@ static int __init prevent_bootmem_remove_init(void)
>  		return ret;
>  
>  	validate_bootmem_online();
> -	ret = register_memory_notifier(&prevent_bootmem_remove_nb);
> +	ret = register_memory_notifier(&prevent_memory_remove_nb);
>  	if (ret)
>  		pr_err("%s: Notifier registration failed %d\n", __func__, ret);
>  
>  	return ret;
>  }
> -early_initcall(prevent_bootmem_remove_init);
> +early_initcall(prevent_memory_remove_init);
>  #endif
>  
>  pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,

Re: [PATCH V2 2/2] arm64/mm: Reject memory removal that splits a kernel leaf mapping

[Anshuman Khandual]

On 10/02/26 3:17 PM, Ryan Roberts wrote:
> On 03/02/2026 13:03, Anshuman Khandual wrote:
>> Linear and vmemmap mapings that get teared down during a memory hot remove
>> operation might contain leaf level entries on any page table level. If the
>> requested memory range's linear or vmemmap mappings falls within such leaf
>> entries, new mappings need to be created for the remaning memory mapped on
>> the leaf entry earlier, following standard break before make aka BBM rules.
>> But kernel cannot tolerate BBM amd hemce remapping to fine grained leaves
> 
> nits:                            ^^^ ^^^^^

Yikes ! will fix the typos.

> 
>> would not be possible on systems without BBML2_NOABORT.
>>
>> Currently memory hot remove operation does not perform such restructuring,
>> and so removing memory ranges that could split a kernel leaf level mapping
>> need to be rejected.
>>
>> while memory_hotplug.c does appear to permit hot removing arbitrary ranges
>> of memory, the higher layers that drive memory_hotplug (e.g. ACPI, virtio,
>> ...) all appear to treat memory as fixed size devices. So it is impossible
>> to hotunplug a different amount than was previously hotplugged, and hence
>> we should never see a rejection in practice, but adding the check makes us
>> robust against a future change.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/
>> Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
>> Cc: stable@vger.kernel.org
> 
> Given your statement in the commit log above about how this never happens in
> practice but is instead to make us robust to any future changes, perhaps this is
> not a stable candidate?
> 
> For clarity; I do agree that this extra robustness is useful so I think we
> should aim to get it upstream, I just don't think it needs to go to stable.

Will drop the `Cc: stable` reference.

> 
>> Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> 
> Other than above nit, LGTM:
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

Thanks ! 

Will respin the series on upcoming - rc1 version.

> 
>> ---
>>  arch/arm64/mm/mmu.c | 155 ++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 149 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 8ec8a287aaa1..3fb9bcbd739a 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -2063,6 +2063,142 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
>>  	__remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size);
>>  }
>>  
>> +
>> +static bool addr_splits_kernel_leaf(unsigned long addr)
>> +{
>> +	pgd_t *pgdp, pgd;
>> +	p4d_t *p4dp, p4d;
>> +	pud_t *pudp, pud;
>> +	pmd_t *pmdp, pmd;
>> +	pte_t *ptep, pte;
>> +
>> +	/*
>> +	 * PGD level:
>> +	 *
>> +	 * If addr is PGD_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, PGDIR_SIZE) == addr)
>> +		return false;
>> +
>> +	pgdp = pgd_offset_k(addr);
>> +	pgd = pgdp_get(pgdp);
>> +	if (!pgd_present(pgd))
>> +		return false;
>> +
>> +	/*
>> +	 * P4D level:
>> +	 *
>> +	 * If addr is P4D_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, P4D_SIZE) == addr)
>> +		return false;
>> +
>> +	p4dp = p4d_offset(pgdp, addr);
>> +	p4d = p4dp_get(p4dp);
>> +	if (!p4d_present(p4d))
>> +		return false;
>> +
>> +	/*
>> +	 * PUD level:
>> +	 *
>> +	 * If addr is PUD_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, PUD_SIZE) == addr)
>> +		return false;
>> +
>> +	pudp = pud_offset(p4dp, addr);
>> +	pud = pudp_get(pudp);
>> +	if (!pud_present(pud))
>> +		return false;
>> +
>> +	if (pud_leaf(pud))
>> +		return true;
>> +
>> +	/*
>> +	 * CONT_PMD level:
>> +	 *
>> +	 * If addr is CONT_PMD_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, CONT_PMD_SIZE) == addr)
>> +		return false;
>> +
>> +	pmdp = pmd_offset(pudp, addr);
>> +	pmd = pmdp_get(pmdp);
>> +	if (!pmd_present(pmd))
>> +		return false;
>> +
>> +	if (pmd_cont(pmd))
>> +		return true;
>> +
>> +	/*
>> +	 * PMD level:
>> +	 *
>> +	 * If addr is PMD_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
>> +		return false;
>> +
>> +	if (pmd_leaf(pmd))
>> +		return true;
>> +
>> +	/*
>> +	 * CONT_PTE level:
>> +	 *
>> +	 * If addr is CONT_PTE_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, CONT_PTE_SIZE) == addr)
>> +		return false;
>> +
>> +	ptep = pte_offset_kernel(pmdp, addr);
>> +	pte = __ptep_get(ptep);
>> +	if (!pte_present(pte))
>> +		return false;
>> +
>> +	if (pte_cont(pte))
>> +		return true;
>> +
>> +	/*
>> +	 * PTE level:
>> +	 *
>> +	 * If addr is PAGE_SIZE aligned - already on a leaf boundary
>> +	 */
>> +	if (ALIGN_DOWN(addr, PAGE_SIZE) == addr)
>> +		return false;
>> +	return true;
>> +}
>> +
>> +static bool can_unmap_without_split(unsigned long pfn, unsigned long nr_pages)
>> +{
>> +	unsigned long phys_start, phys_end, size, start, end;
>> +
>> +	phys_start = PFN_PHYS(pfn);
>> +	phys_end = phys_start + nr_pages * PAGE_SIZE;
>> +
>> +	/*
>> +	 * PFN range's linear map edges are leaf entry aligned
>> +	 */
>> +	start = __phys_to_virt(phys_start);
>> +	end =  __phys_to_virt(phys_end);
>> +	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
>> +		pr_warn("[%lx %lx] splits a leaf entry in linear map\n",
>> +			phys_start, phys_end);
>> +		return false;
>> +	}
>> +
>> +	/*
>> +	 * PFN range's vmemmap edges are leaf entry aligned
>> +	 */
>> +	size = nr_pages * sizeof(struct page);
>> +	start = (unsigned long)pfn_to_page(pfn);
>> +	end = start + size;
>> +	if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) {
>> +		pr_warn("[%lx %lx] splits a leaf entry in vmemmap\n",
>> +			phys_start, phys_end);
>> +		return false;
>> +	}
>> +	return true;
>> +}
>> +
>>  /*
>>   * This memory hotplug notifier helps prevent boot memory from being
>>   * inadvertently removed as it blocks pfn range offlining process in
>> @@ -2071,8 +2207,11 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
>>   * In future if and when boot memory could be removed, this notifier
>>   * should be dropped and free_hotplug_page_range() should handle any
>>   * reserved pages allocated during boot.
>> + *
>> + * This also blocks any memory remove that would have caused a split
>> + * in leaf entry in kernel linear or vmemmap mapping.
>>   */
>> -static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
>> +static int prevent_memory_remove_notifier(struct notifier_block *nb,
>>  					   unsigned long action, void *data)
>>  {
>>  	struct mem_section *ms;
>> @@ -2118,11 +2257,15 @@ static int prevent_bootmem_remove_notifier(struct notifier_block *nb,
>>  			return NOTIFY_DONE;
>>  		}
>>  	}
>> +
>> +	if (!can_unmap_without_split(pfn, arg->nr_pages))
>> +		return NOTIFY_BAD;
>> +
>>  	return NOTIFY_OK;
>>  }
>>  
>> -static struct notifier_block prevent_bootmem_remove_nb = {
>> -	.notifier_call = prevent_bootmem_remove_notifier,
>> +static struct notifier_block prevent_memory_remove_nb = {
>> +	.notifier_call = prevent_memory_remove_notifier,
>>  };
>>  
>>  /*
>> @@ -2172,7 +2315,7 @@ static void validate_bootmem_online(void)
>>  	}
>>  }
>>  
>> -static int __init prevent_bootmem_remove_init(void)
>> +static int __init prevent_memory_remove_init(void)
>>  {
>>  	int ret = 0;
>>  
>> @@ -2180,13 +2323,13 @@ static int __init prevent_bootmem_remove_init(void)
>>  		return ret;
>>  
>>  	validate_bootmem_online();
>> -	ret = register_memory_notifier(&prevent_bootmem_remove_nb);
>> +	ret = register_memory_notifier(&prevent_memory_remove_nb);
>>  	if (ret)
>>  		pr_err("%s: Notifier registration failed %d\n", __func__, ret);
>>  
>>  	return ret;
>>  }
>> -early_initcall(prevent_bootmem_remove_init);
>> +early_initcall(prevent_memory_remove_init);
>>  #endif
>>  
>>  pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
>