mm/zsmalloc: use a proper page type

Let's clean it up: use a proper page type and store our data (offset into
a page) in the lower 16 bit as documented.

We won't be able to support 256 KiB base pages, which is acceptable. 
Teach Kconfig to handle that cleanly using a new CONFIG_HAVE_ZSMALLOC.

Based on this, we should do a proper "struct zsdesc" conversion, as
proposed in [1].

This removes the last _mapcount/page_type offender.

[1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@gmail.com/

Link: https://lkml.kernel.org/r/20240529111904.2069608-4-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>	[zram/zsmalloc workloads]
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/block/zram/Kconfig

@@ -2,6 +2,7 @@
 config ZRAM
 	tristate "Compressed RAM block device support"
 	depends on BLOCK && SYSFS && MMU
+	depends on HAVE_ZSMALLOC
 	depends on CRYPTO_LZO || CRYPTO_ZSTD || CRYPTO_LZ4 || CRYPTO_LZ4HC || CRYPTO_842
 	select ZSMALLOC
 	help

include/linux/page-flags.h

@@ -947,6 +947,7 @@ enum pagetype {
 	PG_guard	= 0x08000000,
 	PG_hugetlb	= 0x04000000,
 	PG_slab		= 0x02000000,
+	PG_zsmalloc	= 0x01000000,
 
 	PAGE_TYPE_BASE	= 0x80000000,
 
@@ -1071,6 +1072,8 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb)
 FOLIO_TEST_FLAG_FALSE(hugetlb)
 #endif
 
+PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
+
 /**
  * PageHuge - Determine if the page belongs to hugetlbfs
  * @page: The page to test.

mm/Kconfig

@@ -128,7 +128,7 @@ config ZSWAP_COMPRESSOR_DEFAULT
 choice
 	prompt "Default allocator"
 	depends on ZSWAP
-	default ZSWAP_ZPOOL_DEFAULT_ZSMALLOC if MMU
+	default ZSWAP_ZPOOL_DEFAULT_ZSMALLOC if HAVE_ZSMALLOC
 	default ZSWAP_ZPOOL_DEFAULT_ZBUD
 	help
 	  Selects the default allocator for the compressed cache for
@@ -154,6 +154,7 @@ config ZSWAP_ZPOOL_DEFAULT_Z3FOLD
 
 config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
 	bool "zsmalloc"
+	depends on HAVE_ZSMALLOC
 	select ZSMALLOC
 	help
 	  Use the zsmalloc allocator as the default allocator.
@@ -186,10 +187,15 @@ config Z3FOLD
 	  page. It is a ZBUD derivative so the simplicity and determinism are
 	  still there.
 
+config HAVE_ZSMALLOC
+	def_bool y
+	depends on MMU
+	depends on PAGE_SIZE_LESS_THAN_256KB # we want <= 64 KiB
+
 config ZSMALLOC
 	tristate
 	prompt "N:1 compression allocator (zsmalloc)" if ZSWAP
-	depends on MMU
+	depends on HAVE_ZSMALLOC
 	help
 	  zsmalloc is a slab-based memory allocator designed to store
 	  pages of various compression levels efficiently. It achieves

mm/zsmalloc.c

@@ -20,7 +20,8 @@
  *	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: first object offset in a subpage of zspage
+ *	page->page_type: PG_zsmalloc, lower 16 bit locate the first object
+ *		offset in a subpage of a zspage
  *
  * Usage of struct page flags:
  *	PG_private: identifies the first component page
@@ -450,14 +451,28 @@ static inline struct page *get_first_page(struct zspage *zspage)
 	return first_page;
 }
 
+#define FIRST_OBJ_PAGE_TYPE_MASK	0xffff
+
+static inline void reset_first_obj_offset(struct page *page)
+{
+	VM_WARN_ON_ONCE(!PageZsmalloc(page));
+	page->page_type |= FIRST_OBJ_PAGE_TYPE_MASK;
+}
+
 static inline unsigned int get_first_obj_offset(struct page *page)
 {
-	return page->page_type;
+	VM_WARN_ON_ONCE(!PageZsmalloc(page));
+	return page->page_type & FIRST_OBJ_PAGE_TYPE_MASK;
 }
 
 static inline void set_first_obj_offset(struct page *page, unsigned int offset)
 {
-	page->page_type = offset;
+	/* With 16 bit available, we can support offsets into 64 KiB pages. */
+	BUILD_BUG_ON(PAGE_SIZE > SZ_64K);
+	VM_WARN_ON_ONCE(!PageZsmalloc(page));
+	VM_WARN_ON_ONCE(offset & ~FIRST_OBJ_PAGE_TYPE_MASK);
+	page->page_type &= ~FIRST_OBJ_PAGE_TYPE_MASK;
+	page->page_type |= offset & FIRST_OBJ_PAGE_TYPE_MASK;
 }
 
 static inline unsigned int get_freeobj(struct zspage *zspage)
@@ -791,8 +806,9 @@ static void reset_page(struct page *page)
 	__ClearPageMovable(page);
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
-	page_mapcount_reset(page);
 	page->index = 0;
+	reset_first_obj_offset(page);
+	__ClearPageZsmalloc(page);
 }
 
 static int trylock_zspage(struct zspage *zspage)
@@ -965,11 +981,13 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
 		if (!page) {
 			while (--i >= 0) {
 				dec_zone_page_state(pages[i], NR_ZSPAGES);
+				__ClearPageZsmalloc(pages[i]);
 				__free_page(pages[i]);
 			}
 			cache_free_zspage(pool, zspage);
 			return NULL;
 		}
+		__SetPageZsmalloc(page);
 
 		inc_zone_page_state(page, NR_ZSPAGES);
 		pages[i] = page;
@@ -1754,6 +1772,9 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 
 	VM_BUG_ON_PAGE(!PageIsolated(page), page);
 
+	/* We're committed, tell the world that this is a Zsmalloc page. */
+	__SetPageZsmalloc(newpage);
+
 	/* The page is locked, so this pointer must remain valid */
 	zspage = get_zspage(page);
 	pool = zspage->pool;