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Date:   Tue, 20 Jun 2023 00:47:54 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
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Subject: [PATCH v2 05/12] powerpc: add pte_free_defer() for pgtables sharing
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Add powerpc-specific 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 precedes
the generic version to avoid build breakage from incompatible pgtable_t.

This is awkward because the struct page contains only one rcu_head, but
that page may be shared between PTE_FRAG_NR pagetables, each wanting to
use the rcu_head at the same time: account concurrent deferrals with a
heightened refcount, only the first making use of the rcu_head, but
re-deferring if more deferrals arrived during its grace period.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 arch/powerpc/include/asm/pgalloc.h |  4 +++
 arch/powerpc/mm/pgtable-frag.c     | 51 ++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/arch/powerpc/include/asm/pgalloc.h b/arch/powerpc/include/asm/pgalloc.h
index 3360cad78ace..3a971e2a8c73 100644
--- a/arch/powerpc/include/asm/pgalloc.h
+++ b/arch/powerpc/include/asm/pgalloc.h
@@ -45,6 +45,10 @@ static inline void pte_free(struct mm_struct *mm, pgtable_t ptepage)
 	pte_fragment_free((unsigned long *)ptepage, 0);
 }
 
+/* arch use pte_free_defer() implementation in arch/powerpc/mm/pgtable-frag.c */
+#define pte_free_defer pte_free_defer
+void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable);
+
 /*
  * Functions that deal with pagetables that could be at any level of
  * the table need to be passed an "index_size" so they know how to
diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
index 20652daa1d7e..e4f58c5fc2ac 100644
--- a/arch/powerpc/mm/pgtable-frag.c
+++ b/arch/powerpc/mm/pgtable-frag.c
@@ -120,3 +120,54 @@ void pte_fragment_free(unsigned long *table, int kernel)
 		__free_page(page);
 	}
 }
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#define PTE_FREE_DEFERRED 0x10000 /* beyond any PTE_FRAG_NR */
+
+static void pte_free_now(struct rcu_head *head)
+{
+	struct page *page;
+	int refcount;
+
+	page = container_of(head, struct page, rcu_head);
+	refcount = atomic_sub_return(PTE_FREE_DEFERRED - 1,
+				     &page->pt_frag_refcount);
+	if (refcount < PTE_FREE_DEFERRED) {
+		pte_fragment_free((unsigned long *)page_address(page), 0);
+		return;
+	}
+	/*
+	 * One page may be shared between PTE_FRAG_NR pagetables.
+	 * At least one more call to pte_free_defer() came in while we
+	 * were already deferring, so the free must be deferred again;
+	 * but just for one grace period, however many calls came in.
+	 */
+	while (refcount >= PTE_FREE_DEFERRED + PTE_FREE_DEFERRED) {
+		refcount = atomic_sub_return(PTE_FREE_DEFERRED,
+					     &page->pt_frag_refcount);
+	}
+	/* Remove that refcount of 1 left for fragment freeing above */
+	atomic_dec(&page->pt_frag_refcount);
+	call_rcu(&page->rcu_head, pte_free_now);
+}
+
+void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
+{
+	struct page *page;
+
+	page = virt_to_page(pgtable);
+	/*
+	 * One page may be shared between PTE_FRAG_NR pagetables: only queue
+	 * it once for freeing, but note whenever the free must be deferred.
+	 *
+	 * (This would be much simpler if the struct page had an rcu_head for
+	 * each fragment, or if we could allocate a separate array for that.)
+	 *
+	 * Convert our refcount of 1 to a refcount of PTE_FREE_DEFERRED, and
+	 * proceed to call_rcu() only when the rcu_head is not already in use.
+	 */
+	if (atomic_add_return(PTE_FREE_DEFERRED - 1, &page->pt_frag_refcount) <
+			      PTE_FREE_DEFERRED + PTE_FREE_DEFERRED)
+		call_rcu(&page->rcu_head, pte_free_now);
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
-- 
2.35.3