f2fs: avoid balanc_fs during evict_inode

1. Background

Previously, if f2fs tries to move data blocks of an *evicting* inode during the
cleaning process, it stops the process incompletely and then restarts the whole
process, since it needs a locked inode to grab victim data pages in its address
space. In order to get a locked inode, iget_locked() by f2fs_iget() is normally
used, but, it waits if the inode is on freeing.

So, here is a deadlock scenario.
1. f2fs_evict_inode()       <- inode "A"
  2. f2fs_balance_fs()
    3. f2fs_gc()
      4. gc_data_segment()
        5. f2fs_iget()      <- inode "A" too!

If step #1 and #5 treat a same inode "A", step #5 would fall into deadlock since
the inode "A" is on freeing. In order to resolve this, f2fs_iget_nowait() which
skips __wait_on_freeing_inode() was introduced in step #5, and stops f2fs_gc()
to complete f2fs_evict_inode().

1. f2fs_evict_inode()           <- inode "A"
  2. f2fs_balance_fs()
    3. f2fs_gc()
      4. gc_data_segment()
        5. f2fs_iget_nowait()   <- inode "A", then stop f2fs_gc() w/ -ENOENT

2. Problem and Solution

In the above scenario, however, f2fs cannot finish f2fs_evict_inode() only if:
 o there are not enough free sections, and
 o f2fs_gc() tries to move data blocks of the *evicting* inode repeatedly.

So, the final solution is to use f2fs_iget() and remove f2fs_balance_fs() in
f2fs_evict_inode().
The f2fs_evict_inode() actually truncates all the data and node blocks, which
means that it doesn't produce any dirty node pages accordingly.
So, we don't need to do f2fs_balance_fs() in practical.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/file.c

@@ -298,8 +298,6 @@ void f2fs_truncate(struct inode *inode)
 		inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 		mark_inode_dirty(inode);
 	}
-
-	f2fs_balance_fs(F2FS_SB(inode->i_sb));
 }
 
 static int f2fs_getattr(struct vfsmount *mnt,
@@ -356,6 +354,7 @@ int f2fs_setattr(struct dentry *dentry, struct iattr *attr)
 			attr->ia_size != i_size_read(inode)) {
 		truncate_setsize(inode, attr->ia_size);
 		f2fs_truncate(inode);
+		f2fs_balance_fs(F2FS_SB(inode->i_sb));
 	}
 
 	__setattr_copy(inode, attr);