[RFC PATCH 3/4] arm64: pause remote CPUs to update vmemmap

[Yu Zhao <yuzhao@google.com>]

Pause remote CPUs so that the local CPU can follow the proper BBM
sequence to safely update the vmemmap mapping `struct page` areas.

While updating the vmemmap, it is guaranteed that neither the local
CPU nor the remote ones will access the `struct page` area being
updated, and therefore they will not trigger kernel PFs.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
 arch/arm64/include/asm/pgalloc.h | 55 ++++++++++++++++++++++++++++++++
 mm/hugetlb_vmemmap.c             | 14 ++++++++
 2 files changed, 69 insertions(+)

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index 8ff5f2a2579e..1af1aa34a351 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h
@@ -12,6 +12,7 @@
 #include <asm/processor.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
+#include <asm/cpu.h>
 
 #define __HAVE_ARCH_PGD_FREE
 #define __HAVE_ARCH_PUD_FREE
@@ -137,4 +138,58 @@ pmd_populate(struct mm_struct *mm, pmd_t *pmdp, pgtable_t ptep)
 	__pmd_populate(pmdp, page_to_phys(ptep), PMD_TYPE_TABLE | PMD_TABLE_PXN);
 }
 
+#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
+
+#define vmemmap_update_lock vmemmap_update_lock
+static inline void vmemmap_update_lock(void)
+{
+	cpus_read_lock();
+}
+
+#define vmemmap_update_unlock vmemmap_update_unlock
+static inline void vmemmap_update_unlock(void)
+{
+	cpus_read_unlock();
+}
+
+#define vmemmap_update_pte vmemmap_update_pte
+static inline void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte)
+{
+	preempt_disable();
+	pause_remote_cpus();
+
+	pte_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+	set_pte_at(&init_mm, addr, ptep, pte);
+
+	resume_remote_cpus();
+	preempt_enable();
+}
+
+#define vmemmap_update_pmd vmemmap_update_pmd
+static inline void vmemmap_update_pmd(unsigned long addr, pmd_t *pmdp, pte_t *ptep)
+{
+	preempt_disable();
+	pause_remote_cpus();
+
+	pmd_clear(pmdp);
+	flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+	pmd_populate_kernel(&init_mm, pmdp, ptep);
+
+	resume_remote_cpus();
+	preempt_enable();
+}
+
+#define vmemmap_flush_tlb_all vmemmap_flush_tlb_all
+static inline void vmemmap_flush_tlb_all(void)
+{
+}
+
+#define vmemmap_flush_tlb_range vmemmap_flush_tlb_range
+static inline void vmemmap_flush_tlb_range(unsigned long start, unsigned long end)
+{
+}
+
+#endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
+
 #endif
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 2dd92e58f304..893c73493d9c 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -46,6 +46,18 @@ struct vmemmap_remap_walk {
 	unsigned long		flags;
 };
 
+#ifndef vmemmap_update_lock
+static void vmemmap_update_lock(void)
+{
+}
+#endif
+
+#ifndef vmemmap_update_unlock
+static void vmemmap_update_unlock(void)
+{
+}
+#endif
+
 #ifndef vmemmap_update_pmd
 static inline void vmemmap_update_pmd(unsigned long addr,
 				      pmd_t *pmdp, pte_t *ptep)
@@ -194,10 +206,12 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
 
 	VM_BUG_ON(!PAGE_ALIGNED(start | end));
 
+	vmemmap_update_lock();
 	mmap_read_lock(&init_mm);
 	ret = walk_page_range_novma(&init_mm, start, end, &vmemmap_remap_ops,
 				    NULL, walk);
 	mmap_read_unlock(&init_mm);
+	vmemmap_update_unlock();
 	if (ret)
 		return ret;
 
-- 
2.46.0.rc2.264.g509ed76dc8-goog