ext4: remove bdev_try_to_free_page() callback

After we introduce a jbd2 shrinker to release checkpointed buffer's
journal head, we could free buffer without bdev_try_to_free_page()
under memory pressure. So this patch remove the whole
bdev_try_to_free_page() callback directly. It also remove many
use-after-free issues relate to it together.
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210610112440.3438139-8-yi.zhang@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -1442,26 +1442,6 @@ static int ext4_nfs_commit_metadata(struct inode *inode)
 	return ext4_write_inode(inode, &wbc);
 }
 
-/*
- * Try to release metadata pages (indirect blocks, directories) which are
- * mapped via the block device.  Since these pages could have journal heads
- * which would prevent try_to_free_buffers() from freeing them, we must use
- * jbd2 layer's try_to_free_buffers() function to release them.
- */
-static int bdev_try_to_free_page(struct super_block *sb, struct page *page,
-				 gfp_t wait)
-{
-	journal_t *journal = EXT4_SB(sb)->s_journal;
-
-	WARN_ON(PageChecked(page));
-	if (!page_has_buffers(page))
-		return 0;
-	if (journal)
-		return jbd2_journal_try_to_free_buffers(journal, page);
-
-	return try_to_free_buffers(page);
-}
-
 #ifdef CONFIG_FS_ENCRYPTION
 static int ext4_get_context(struct inode *inode, void *ctx, size_t len)
 {
@@ -1656,7 +1636,6 @@ static const struct super_operations ext4_sops = {
 	.quota_write	= ext4_quota_write,
 	.get_dquots	= ext4_get_dquots,
 #endif
-	.bdev_try_to_free_page = bdev_try_to_free_page,
 };
 
 static const struct export_operations ext4_export_ops = {