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Date:   Tue, 11 Jul 2023 21:39:48 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
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Subject: [PATCH v3 08/13] mm/pgtable: add pte_free_defer() for pgtable as
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Add the generic pte_free_defer(), to call pte_free() via call_rcu().
pte_free_defer() will be called inside khugepaged's retract_page_tables()
loop, where allocating extra memory cannot be relied upon.  This version
suits all those architectures which use an unfragmented page for one page
table (none of whose pte_free()s use the mm arg which was passed to it).

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 include/linux/mm_types.h |  4 ++++
 include/linux/pgtable.h  |  2 ++
 mm/pgtable-generic.c     | 20 ++++++++++++++++++++
 3 files changed, 26 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index de10fc797c8e..17a7868f00bd 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -144,6 +144,10 @@ struct page {
 		struct {	/* Page table pages */
 			unsigned long _pt_pad_1;	/* compound_head */
 			pgtable_t pmd_huge_pte; /* protected by page->ptl */
+			/*
+			 * A PTE page table page might be freed by use of
+			 * rcu_head: which overlays those two fields above.
+			 */
 			unsigned long _pt_pad_2;	/* mapping */
 			union {
 				struct mm_struct *pt_mm; /* x86 pgds only */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 7f2db400f653..9fa34be65159 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -112,6 +112,8 @@ static inline void pte_unmap(pte_t *pte)
 }
 #endif
 
+void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable);
+
 /* Find an entry in the second-level page table.. */
 #ifndef pmd_offset
 static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address)
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index b9a0c2137cc1..fa9d4d084291 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -13,6 +13,7 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/mm_inline.h>
+#include <asm/pgalloc.h>
 #include <asm/tlb.h>
 
 /*
@@ -230,6 +231,25 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,
 	return pmd;
 }
 #endif
+
+/* arch define pte_free_defer in asm/pgalloc.h for its own implementation */
+#ifndef pte_free_defer
+static void pte_free_now(struct rcu_head *head)
+{
+	struct page *page;
+
+	page = container_of(head, struct page, rcu_head);
+	pte_free(NULL /* mm not passed and not used */, (pgtable_t)page);
+}
+
+void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
+{
+	struct page *page;
+
+	page = pgtable;
+	call_rcu(&page->rcu_head, pte_free_now);
+}
+#endif /* pte_free_defer */
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #if defined(CONFIG_GUP_GET_PXX_LOW_HIGH) && \
-- 
2.35.3