[PATCH v4] riscv: mm: Avoid spurious fault after hotplugging vmemmap

[Vivian Wang]

section_activate() does not flush TLB after populating new vmemmap
pages. On most architectures, this is okay. However it is a problem on
RISC-V since there the TLB caching non-present entries is permitted,
which causes spurious faults on some hardwares.

This seems to be most easily reproduced with DEBUG_VM=y and
PAGE_POISONING=y, which causes these newly mapped struct pages to be
poisoned i.e. written to immediately after mapping.

Add a hook vmemmap_populate_finalize() in __populate_section_memmap()
after population, to allow architectures to handle such situations as
needed. Then implement it on RISC-V to arrange for the existing
exception handler code to deal with these faults if they happen.

Signed-off-by: Vivian Wang <wangruikang@xxxxxxxxxxx>
---
Changes in v4:
- Rebase on v7.2-rc1, drop dependencies
- (No code changes otherwise)
- (A concurrency fix for mark_new_valid_map was sent independently)
  https://lore.kernel.org/linux-riscv/20260629-riscv-mm-new-valid-map-ordering-v1-1-60d8c10c6292@xxxxxxxxxxx/
- Link to v3: https://patch.msgid.link/20260605-mark-after-vmemmap-populate-v3-1-a06001ac9264@xxxxxxxxxxx

Changes in v3:
- Merged back into one patch (Mike)
- (No code changes otherwise.)
- Link to v2: https://patch.msgid.link/20260604-mark-after-vmemmap-populate-v2-0-ab6a7d03b434@xxxxxxxxxxx

Changes in v2:
- Split patch in two, hook point and riscv hook 
- Explain hook necessity in patch 1 message (Mike)
- Make hook #define based (Mike)
- Call finalize hook only on populate success
- Link to v1: https://patch.msgid.link/20260525-mark-after-vmemmap-populate-v1-1-e698d859ba16@xxxxxxxxxxx
---
 arch/riscv/include/asm/pgtable.h | 4 ++++
 arch/riscv/mm/init.c             | 6 ++++++
 mm/sparse-vmemmap.c              | 8 ++++++++
 3 files changed, 18 insertions(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 5d5756bda82e..6b000c990ba7 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -1253,6 +1253,10 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
 #define TASK_SIZE	FIXADDR_START
 #endif
 
+/* Needed on SPARSEMEM_VMEMMAP */
+#define vmemmap_populate_finalize vmemmap_populate_finalize
+void __meminit vmemmap_populate_finalize(void);
+
 #else /* CONFIG_MMU */
 
 #define PAGE_SHARED		__pgprot(0)
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 5b1b3c88b4d1..800cb5c007d1 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1372,6 +1372,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 	 */
 	return vmemmap_populate_hugepages(start, end, node, altmap);
 }
+
+void __meminit vmemmap_populate_finalize(void)
+{
+	/* Avoid faults on cached non-present TLB entries. */
+	mark_new_valid_map();
+}
 #endif
 
 #if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 99e2be39671b..290cafcfd723 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -544,6 +544,12 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 
 #endif
 
+#ifndef vmemmap_populate_finalize
+static void __meminit vmemmap_populate_finalize(void)
+{
+}
+#endif
+
 struct page * __meminit __populate_section_memmap(unsigned long pfn,
 		unsigned long nr_pages, int nid, struct vmem_altmap *altmap,
 		struct dev_pagemap *pgmap)
@@ -564,6 +570,8 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
 	if (r < 0)
 		return NULL;
 
+	vmemmap_populate_finalize();
+
 	return pfn_to_page(pfn);
 }
 

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260525-mark-after-vmemmap-populate-68bd790839c9

Best regards,
--  
Vivian "dramforever" Wang