btrfs: migrate to use folio private instead of page private

As a cleanup and preparation for future folio migration, this patch
would replace all page->private to folio version.  This includes:

- PagePrivate()
  -> folio_test_private()

- page->private
  -> folio_get_private()

- attach_page_private()
  -> folio_attach_private()

- detach_page_private()
  -> folio_detach_private()

Since we're here, also remove the forced cast on page->private, since
it's (void *) already, we don't really need to do the cast.

For now even if we missed some call sites, it won't cause any problem
yet, as we're only using order 0 folio (single page), thus all those
folio/page flags should be synced.

But for the future conversion to utilize higher order folio, the page
<-> folio flag sync is no longer guaranteed, thus we have to migrate to
utilize folio flags.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.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/extent_io.h

@@ -43,10 +43,10 @@ enum {
 };
 
 /*
- * page->private values.  Every page that is controlled by the extent
- * map has page->private set to one.
+ * Folio private values.  Every page that is controlled by the extent map has
+ * folio private set to this value.
  */
-#define EXTENT_PAGE_PRIVATE 1
+#define EXTENT_FOLIO_PRIVATE			1
 
 /*
  * The extent buffer bitmap operations are done with byte granularity instead of

fs/btrfs/file.c

@@ -869,9 +869,9 @@ static int prepare_uptodate_page(struct inode *inode,
 		 * released.
 		 *
 		 * The private flag check is essential for subpage as we need
-		 * to store extra bitmap using page->private.
+		 * to store extra bitmap using folio private.
 		 */
-		if (page->mapping != inode->i_mapping || !PagePrivate(page)) {
+		if (page->mapping != inode->i_mapping || !folio_test_private(folio)) {
 			unlock_page(page);
 			return -EAGAIN;
 		}

fs/btrfs/inode.c

@@ -4725,7 +4725,7 @@ int btrfs_truncate_block(struct btrfs_inode *inode, loff_t from, loff_t len,
 	/*
 	 * We unlock the page after the io is completed and then re-lock it
 	 * above.  release_folio() could have come in between that and cleared
-	 * PagePrivate(), but left the page in the mapping.  Set the page mapped
+	 * folio private, but left the page in the mapping.  Set the page mapped
 	 * here to make sure it's properly set for the subpage stuff.
 	 */
 	ret = set_page_extent_mapped(page);
@@ -7851,13 +7851,14 @@ static void btrfs_readahead(struct readahead_control *rac)
 static void wait_subpage_spinlock(struct page *page)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(page->mapping->host->i_sb);
+	struct folio *folio = page_folio(page);
 	struct btrfs_subpage *subpage;
 
 	if (!btrfs_is_subpage(fs_info, page))
 		return;
 
-	ASSERT(PagePrivate(page) && page->private);
-	subpage = (struct btrfs_subpage *)page->private;
+	ASSERT(folio_test_private(folio) && folio_get_private(folio));
+	subpage = folio_get_private(folio);
 
 	/*
 	 * This may look insane as we just acquire the spinlock and release it,