[PATCH v2] f2fs: skip f2fs_preallocate_blocks() for overwrite case

[Chao Yu]

There is potential hangtask happened during swapfile's writeback:

- loop_kthread_worker_fn		- do_checkpoint
  - kthread_worker_fn
   - loop_queue_work
    - lo_rw_aio
     - f2fs_file_write_iter
      - f2fs_preallocate_blocks
       - f2fs_map_blocks
					 - down_write
        - down_read
         - rwsem_down_read_slowpath
          - schedule

One cause is f2fs_preallocate_blocks() will always be called no matter
the physical block addresses are allocated or not.

This patch tries to check whether block addresses are all allocated with
i_size and i_blocks of inode, it's rough because blocks can be allocated
beyond i_size, however, we can afford skipping block preallocation in this
condition since it's not necessary to do preallocation all the time.

Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
---
v2:
- check overwrite case with i_size and i_blocks roughly.
 fs/f2fs/file.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index cfdc41f87f5d..09565d10611d 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -4390,6 +4390,16 @@ static int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *iter,
 	int flag;
 	int ret;
 
+	/*
+	 * It tries to check whether block addresses are all allocated,
+	 * it's rough because blocks can be allocated beyond i_size,
+	 * however, we can afford skipping block preallocation since
+	 * it's not necessary all the time.
+	 */
+	if (F2FS_BLK_ALIGN(i_size_read(inode)) ==
+			SECTOR_TO_BLOCK(inode->i_blocks))
+		return 0;
+
 	/* If it will be an out-of-place direct write, don't bother. */
 	if (dio && f2fs_lfs_mode(sbi))
 		return 0;
-- 
2.32.0