[RFC PATCH 01/16] mm: add pagechain container for storing multiple pages.

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

When depositing page table pages for 1GB THPs, we need 512 PTE pages +
1 PMD page. Instead of counting and depositing 513 pages, we can use the
PMD page as a leader page and chain the rest 512 PTE pages with ->lru.
This, however, prevents us depositing PMD pages with ->lru, which is
currently used by depositing PTE pages for 2MB THPs. So add a new
pagechain container for PMD pages.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/pagechain.h | 73 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 include/linux/pagechain.h

diff --git a/include/linux/pagechain.h b/include/linux/pagechain.h
new file mode 100644
index 000000000000..be536142b413
--- /dev/null
+++ b/include/linux/pagechain.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * include/linux/pagechain.h
+ *
+ * In many places it is efficient to batch an operation up against multiple
+ * pages. A pagechain is a multipage container which is used for that.
+ */
+
+#ifndef _LINUX_PAGECHAIN_H
+#define _LINUX_PAGECHAIN_H
+
+#include <linux/slab.h>
+
+/* 14 pointers + two long's align the pagechain structure to a power of two */
+#define PAGECHAIN_SIZE	13
+
+struct page;
+
+struct pagechain {
+	struct list_head list;
+	unsigned int nr;
+	struct page *pages[PAGECHAIN_SIZE];
+};
+
+static inline void pagechain_init(struct pagechain *pchain)
+{
+	pchain->nr = 0;
+	INIT_LIST_HEAD(&pchain->list);
+}
+
+static inline void pagechain_reinit(struct pagechain *pchain)
+{
+	pchain->nr = 0;
+}
+
+static inline unsigned int pagechain_count(struct pagechain *pchain)
+{
+	return pchain->nr;
+}
+
+static inline unsigned int pagechain_space(struct pagechain *pchain)
+{
+	return PAGECHAIN_SIZE - pchain->nr;
+}
+
+static inline bool pagechain_empty(struct pagechain *pchain)
+{
+	return pchain->nr == 0;
+}
+
+/*
+ * Add a page to a pagechain.  Returns the number of slots still available.
+ */
+static inline unsigned int pagechain_deposit(struct pagechain *pchain, struct page *page)
+{
+	VM_BUG_ON(!pagechain_space(pchain));
+	pchain->pages[pchain->nr++] = page;
+	return pagechain_space(pchain);
+}
+
+static inline struct page *pagechain_withdraw(struct pagechain *pchain)
+{
+	if (!pagechain_count(pchain))
+		return NULL;
+	return pchain->pages[--pchain->nr];
+}
+
+void __init pagechain_cache_init(void);
+struct pagechain *pagechain_alloc(void);
+void pagechain_free(struct pagechain *pchain);
+
+#endif /* _LINUX_PAGECHAIN_H */
+
-- 
2.28.0