btrfs: introduce the skeleton of btrfs_subpage structure

For sectorsize < page size support, we need a structure to record extra
status info for each sector of a page.

Introduce the skeleton structure, all subpage related code would go to
subpage.[ch].

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/Makefile

@@ -27,7 +27,8 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
 	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
 	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
 	   uuid-tree.o props.o free-space-tree.o tree-checker.o space-info.o \
-	   block-rsv.o delalloc-space.o block-group.o discard.o reflink.o
+	   block-rsv.o delalloc-space.o block-group.o discard.o reflink.o \
+	   subpage.o
 
 btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
 btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o

fs/btrfs/subpage.c

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/slab.h>
+#include "ctree.h"
+#include "subpage.h"
+
+int btrfs_attach_subpage(const struct btrfs_fs_info *fs_info,
+			 struct page *page, enum btrfs_subpage_type type)
+{
+	struct btrfs_subpage *subpage;
+
+	/*
+	 * We have cases like a dummy extent buffer page, which is not mappped
+	 * and doesn't need to be locked.
+	 */
+	if (page->mapping)
+		ASSERT(PageLocked(page));
+	/* Either not subpage, or the page already has private attached */
+	if (fs_info->sectorsize == PAGE_SIZE || PagePrivate(page))
+		return 0;
+
+	subpage = kzalloc(sizeof(struct btrfs_subpage), GFP_NOFS);
+	if (!subpage)
+		return -ENOMEM;
+
+	spin_lock_init(&subpage->lock);
+	attach_page_private(page, subpage);
+	return 0;
+}
+
+void btrfs_detach_subpage(const struct btrfs_fs_info *fs_info,
+			  struct page *page)
+{
+	struct btrfs_subpage *subpage;
+
+	/* Either not subpage, or already detached */
+	if (fs_info->sectorsize == PAGE_SIZE || !PagePrivate(page))
+		return;
+
+	subpage = (struct btrfs_subpage *)detach_page_private(page);
+	ASSERT(subpage);
+	kfree(subpage);
+}

fs/btrfs/subpage.h

@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef BTRFS_SUBPAGE_H
+#define BTRFS_SUBPAGE_H
+
+#include <linux/spinlock.h>
+
+/*
+ * Maximum page size we support is 64K, minimum sector size is 4K, u16 bitmap
+ * is sufficient. Regular bitmap_* is not used due to size reasons.
+ */
+#define BTRFS_SUBPAGE_BITMAP_SIZE	16
+
+/*
+ * Structure to trace status of each sector inside a page, attached to
+ * page::private for both data and metadata inodes.
+ */
+struct btrfs_subpage {
+	/* Common members for both data and metadata pages */
+	spinlock_t lock;
+};
+
+enum btrfs_subpage_type {
+	BTRFS_SUBPAGE_METADATA,
+	BTRFS_SUBPAGE_DATA,
+};
+
+int btrfs_attach_subpage(const struct btrfs_fs_info *fs_info,
+			 struct page *page, enum btrfs_subpage_type type);
+void btrfs_detach_subpage(const struct btrfs_fs_info *fs_info,
+			  struct page *page);
+
+#endif

fs/btrfs/super.c

@@ -48,7 +48,6 @@
 #include "tests/btrfs-tests.h"
 #include "block-group.h"
 #include "discard.h"
-
 #include "qgroup.h"
 #define CREATE_TRACE_POINTS
 #include <trace/events/btrfs.h>