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@xxxxxxxxxx>
---
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
