[PATCH v4 03/18] btrfs: introduce the skeleton of btrfs_subpage structure

[Qu Wenruo <wqu@suse.com>]

For btrfs subpage support, we need a structure to record extra info for
the status of each sectors of a page.

This patch will introduce the skeleton structure for future btrfs
subpage support.
All subpage related code would go to subpage.[ch] to avoid populating
the existing code base.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/Makefile  |  3 ++-
 fs/btrfs/subpage.c | 39 +++++++++++++++++++++++++++++++++++++++
 fs/btrfs/subpage.h | 31 +++++++++++++++++++++++++++++++
 3 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 fs/btrfs/subpage.c
 create mode 100644 fs/btrfs/subpage.h

diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index 9f1b1a88e317..942562e11456 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -11,7 +11,8 @@ 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
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
new file mode 100644
index 000000000000..c6ab32db3995
--- /dev/null
+++ b/fs/btrfs/subpage.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "subpage.h"
+
+int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page)
+{
+	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(*subpage), GFP_NOFS);
+	if (!subpage)
+		return -ENOMEM;
+
+	spin_lock_init(&subpage->lock);
+	attach_page_private(page, subpage);
+	return 0;
+}
+
+void btrfs_detach_subpage(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);
+}
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
new file mode 100644
index 000000000000..96f3b226913e
--- /dev/null
+++ b/fs/btrfs/subpage.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef BTRFS_SUBPAGE_H
+#define BTRFS_SUBPAGE_H
+
+#include <linux/spinlock.h>
+#include "ctree.h"
+
+/*
+ * Since the maximum page size btrfs is going to support is 64K while the
+ * minimum sectorsize is 4K, this means a u16 bitmap is enough.
+ *
+ * The regular bitmap requires 32 bits as minimal bitmap size, so we can't use
+ * existing bitmap_* helpers here.
+ */
+#define BTRFS_SUBPAGE_BITMAP_SIZE	16
+
+/*
+ * Structure to trace status of each sector inside a page.
+ *
+ * Will be attached to page::private for both data and metadata inodes.
+ */
+struct btrfs_subpage {
+	/* Common members for both data and metadata pages */
+	spinlock_t lock;
+};
+
+int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
+void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
+
+#endif /* BTRFS_SUBPAGE_H */
-- 
2.30.0