[PATCH v9 mm-unstable 01/18] mm/zsmalloc: add zpdesc memory descriptor for zswap.zpool

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

From: Alex Shi <alexs@kernel.org>

The 1st patch introduces new memory descriptor zpdesc and renames
zspage.first_page to zspage.first_zpdesc, with no functional change.

We removed the comment about PG_owner_priv_1 since it is no longer used
after commit a41ec880aa7b ("zsmalloc: move huge compressed obj from page
to zspage").

[42.hyeyoo: rework comments a little bit]
Originally-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Alex Shi <alexs@kernel.org>
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
 mm/zpdesc.h   | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/zsmalloc.c |  28 +++----------
 2 files changed, 112 insertions(+), 23 deletions(-)
 create mode 100644 mm/zpdesc.h

diff --git a/mm/zpdesc.h b/mm/zpdesc.h
new file mode 100644
index 000000000000..e0852498aecf
--- /dev/null
+++ b/mm/zpdesc.h
@@ -0,0 +1,107 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* zpdesc.h: zswap.zpool memory descriptor
+ *
+ * Written by Alex Shi <alexs@kernel.org>
+ *	      Hyeonggon Yoo <42.hyeyoo@gmail.com>
+ */
+#ifndef __MM_ZPDESC_H__
+#define __MM_ZPDESC_H__
+
+/*
+ * struct zpdesc -	Memory descriptor for zpool memory.
+ * @flags:		Page flags, mostly unused by zsmalloc.
+ * @lru:		Indirectly used by page migration.
+ * @movable_ops:	Used by page migration.
+ * @next:		Next zpdesc in a zspage in zsmalloc zpool.
+ * @handle:		For huge zspage in zsmalloc zpool.
+ * @zspage:		Points to the zspage this zpdesc is a part of.
+ * @first_obj_offset:	First object offset in zsmalloc zpool.
+ * @_refcount:		The number of references to this zpdesc.
+ *
+ * This struct overlays struct page for now. Do not modify without a good
+ * understanding of the issues. In particular, do not expand into the overlap
+ * with memcg_data.
+ *
+ * Page flags used:
+ * * PG_private identifies the first component page.
+ * * PG_locked is used by page migration code.
+ */
+struct zpdesc {
+	unsigned long flags;
+	struct list_head lru;
+	unsigned long movable_ops;
+	union {
+		struct zpdesc *next;
+		unsigned long handle;
+	};
+	struct zspage *zspage;
+	/*
+	 * Only the lower 24 bits are available for offset, limiting a page
+	 * to 16 MiB. The upper 8 bits are reserved for PGTY_zsmalloc.
+	 *
+	 * Do not access this field directly.
+	 * Instead, use {get,set}_first_obj_offset() helpers.
+	 */
+	unsigned int first_obj_offset;
+	atomic_t _refcount;
+};
+#define ZPDESC_MATCH(pg, zp) \
+	static_assert(offsetof(struct page, pg) == offsetof(struct zpdesc, zp))
+
+ZPDESC_MATCH(flags, flags);
+ZPDESC_MATCH(lru, lru);
+ZPDESC_MATCH(mapping, movable_ops);
+ZPDESC_MATCH(index, next);
+ZPDESC_MATCH(index, handle);
+ZPDESC_MATCH(private, zspage);
+ZPDESC_MATCH(page_type, first_obj_offset);
+ZPDESC_MATCH(_refcount, _refcount);
+#undef ZPDESC_MATCH
+static_assert(sizeof(struct zpdesc) <= sizeof(struct page));
+
+/*
+ * zpdesc_page - The first struct page allocated for a zpdesc
+ * @zp: The zpdesc.
+ *
+ * A convenience wrapper for converting zpdesc to the first struct page of the
+ * underlying folio, to communicate with code not yet converted to folio or
+ * struct zpdesc.
+ *
+ */
+#define zpdesc_page(zp)			(_Generic((zp),			\
+	const struct zpdesc *:		(const struct page *)(zp),	\
+	struct zpdesc *:		(struct page *)(zp)))
+
+/**
+ * zpdesc_folio - The folio allocated for a zpdesc
+ * @zpdesc: The zpdesc.
+ *
+ * Zpdescs are descriptors for zpool memory. The zpool memory itself is
+ * allocated as folios that contain the zpool objects, and zpdesc uses specific
+ * fields in the first struct page of the folio - those fields are now accessed
+ * by struct zpdesc.
+ *
+ * It is occasionally necessary convert to back to a folio in order to
+ * communicate with the rest of the mm. Please use this helper function
+ * instead of casting yourself, as the implementation may change in the future.
+ */
+#define zpdesc_folio(zp)		(_Generic((zp),			\
+	const struct zpdesc *:		(const struct folio *)(zp),	\
+	struct zpdesc *:		(struct folio *)(zp)))
+/**
+ * page_zpdesc - Converts from first struct page to zpdesc.
+ * @p: The first (either head of compound or single) page of zpdesc.
+ *
+ * A temporary wrapper to convert struct page to struct zpdesc in situations
+ * where we know the page is the compound head, or single order-0 page.
+ *
+ * Long-term ideally everything would work with struct zpdesc directly or go
+ * through folio to struct zpdesc.
+ *
+ * Return: The zpdesc which contains this page
+ */
+#define page_zpdesc(p)			(_Generic((p),			\
+	const struct page *:		(const struct zpdesc *)(p),	\
+	struct page *:			(struct zpdesc *)(p)))
+
+#endif
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 64b66a4d3e6e..00d111f011be 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -13,24 +13,6 @@
  * Released under the terms of GNU General Public License Version 2.0
  */
 
-/*
- * Following is how we use various fields and flags of underlying
- * struct page(s) to form a zspage.
- *
- * Usage of struct page fields:
- *	page->private: points to zspage
- *	page->index: links together all component pages of a zspage
- *		For the huge page, this is always 0, so we use this field
- *		to store handle.
- *	page->page_type: PGTY_zsmalloc, lower 24 bits locate the first object
- *		offset in a subpage of a zspage
- *
- * Usage of struct page flags:
- *	PG_private: identifies the first component page
- *	PG_owner_priv_1: identifies the huge component page
- *
- */
-
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 /*
@@ -67,6 +49,7 @@
 #include <linux/pagemap.h>
 #include <linux/fs.h>
 #include <linux/local_lock.h>
+#include "zpdesc.h"
 
 #define ZSPAGE_MAGIC	0x58
 
@@ -254,7 +237,7 @@ struct zspage {
 	};
 	unsigned int inuse;
 	unsigned int freeobj;
-	struct page *first_page;
+	struct zpdesc *first_zpdesc;
 	struct list_head list; /* fullness list */
 	struct zs_pool *pool;
 	rwlock_t lock;
@@ -459,7 +442,7 @@ static inline void mod_zspage_inuse(struct zspage *zspage, int val)
 
 static inline struct page *get_first_page(struct zspage *zspage)
 {
-	struct page *first_page = zspage->first_page;
+	struct page *first_page = zpdesc_page(zspage->first_zpdesc);
 
 	VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
 	return first_page;
@@ -952,7 +935,7 @@ static void create_page_chain(struct size_class *class, struct zspage *zspage,
 		set_page_private(page, (unsigned long)zspage);
 		page->index = 0;
 		if (i == 0) {
-			zspage->first_page = page;
+			zspage->first_zpdesc = page_zpdesc(page);
 			SetPagePrivate(page);
 			if (unlikely(class->objs_per_zspage == 1 &&
 					class->pages_per_zspage == 1))
@@ -1317,8 +1300,7 @@ static unsigned long obj_malloc(struct zs_pool *pool,
 		/* record handle in the header of allocated chunk */
 		link->handle = handle | OBJ_ALLOCATED_TAG;
 	else
-		/* record handle to page->index */
-		zspage->first_page->index = handle | OBJ_ALLOCATED_TAG;
+		zspage->first_zpdesc->handle = handle | OBJ_ALLOCATED_TAG;
 
 	kunmap_local(vaddr);
 	mod_zspage_inuse(zspage, 1);
-- 
2.43.5