[PATCH v5] mm/sparse-vmemmap: flush_cache_vmap() 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.

Extend the RISC-V flush_cache_vmap() to also handle the vmemmap range,
and call it after hotplugging vmemmap, which gets the possible spurious
fault handled in the exception handler.

At least for now, the only other architecture with both
SPARSEMEM_VMEMMAP and flush_cache_vmap() is PowerPC, which has a similar
problem with newly valid PTEs. But there flush_cache_vmap() is just a
ptesync. So it should be safe to do this for generic code while having
minimal performance impact.

Suggested-by: Muchun Song <muchun.song at linux.dev>
Signed-off-by: Vivian Wang <wangruikang at iscas.ac.cn>
---
Changes in v5:
- Remove hook, use existing flush_cache_vmap() (Muchun)
- Link to v4: https://patch.msgid.link/20260630-mark-after-vmemmap-populate-v4-1-febbc15da028@iscas.ac.cn

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@iscas.ac.cn/
- Link to v3: https://patch.msgid.link/20260605-mark-after-vmemmap-populate-v3-1-a06001ac9264@iscas.ac.cn

(See v3 link for older changes)
---
 arch/riscv/include/asm/cacheflush.h | 3 ++-
 arch/riscv/mm/init.c                | 1 +
 mm/sparse-vmemmap.c                 | 2 ++
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
index 8cfe59483a8f..e935db27f09f 100644
--- a/arch/riscv/include/asm/cacheflush.h
+++ b/arch/riscv/include/asm/cacheflush.h
@@ -56,7 +56,8 @@ static inline void mark_new_valid_map(void)
 #define flush_cache_vmap flush_cache_vmap
 static inline void flush_cache_vmap(unsigned long start, unsigned long end)
 {
-	if (is_vmalloc_or_module_addr((void *)start))
+	if (is_vmalloc_or_module_addr((void *)start) ||
+	    (start >= VMEMMAP_START && end <= VMEMMAP_END))
 		mark_new_valid_map();
 }
 #define flush_cache_vmap_early(start, end)	local_flush_tlb_kernel_range(start, end)
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 5b1b3c88b4d1..1b4f16704133 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -24,6 +24,7 @@
 #include <linux/execmem.h>
 
 #include <asm/alternative.h>
+#include <asm/cacheflush.h>
 #include <asm/fixmap.h>
 #include <asm/io.h>
 #include <asm/kasan.h>
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 99e2be39671b..ebd3ac997f64 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -564,6 +564,8 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
 	if (r < 0)
 		return NULL;
 
+	flush_cache_vmap(start, end);
+
 	return pfn_to_page(pfn);
 }
 

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

Best regards,
--  
Vivian "dramforever" Wang