[PATCH 2/2] hfs: release preallocated blocks in hfs_file_truncate()

[Viacheslav Dubeyko]

hfs_get_block() extends a fork through hfs_extend_file(),
which allocates whole clumps. Also, hfs_inode_read_fork()
initializes alloc_blocks from the on-disk physical EOF,
while i_size and phys_size are set to the logical EOF.
As a result, alloc_blocks can be ahead of the number of
blocks really needed to hold i_size, even when phys_size
matches i_size.

hfs_file_truncate() returns early whenever i_size equals
phys_size, before it compares the required block count with
alloc_blocks. So, for example, a file whose size is aligned
on the block size keeps its clump preallocation after the
last close. Moreover, hfs_cat_delete() does not free the data
fork and relies on hfs_delete_inode() truncating the file to
zero size. If a file has zero logical size but allocated
blocks, the early return skips the truncation, and its blocks
remain marked as used in the volume bitmap after the file has
been deleted.

Compute the required block count first and return early only
when neither the size nor the allocation needs to be trimmed.
This keeps the intent of the original early return of not
dirtying an unchanged inode. The existing logic further down
already handles the case of blk_cnt being equal to alloc_blocks.

Fixes: f76d28d235cf ("[PATCH] hfs: don't dirty unchanged inode")
cc: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx>
cc: Yangtao Li <frank.li@xxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
Signed-off-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>
---
 fs/hfs/extent.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/hfs/extent.c b/fs/hfs/extent.c
index 26357086517f..42231618f3f1 100644
--- a/fs/hfs/extent.c
+++ b/fs/hfs/extent.c
@@ -507,11 +507,20 @@ void hfs_file_truncate(struct inode *inode)
 		if (res)
 			inode->i_size = HFS_I(inode)->phys_size;
 		return;
-	} else if (inode->i_size == HFS_I(inode)->phys_size)
-		return;
+	}
+
 	size = inode->i_size + HFS_SB(sb)->alloc_blksz - 1;
 	blk_cnt = size / HFS_SB(sb)->alloc_blksz;
 	alloc_cnt = HFS_I(inode)->alloc_blocks;
+
+	/*
+	 * hfs_extend_file() allocates whole clumps, so alloc_blocks can
+	 * be ahead of phys_size even when phys_size matches i_size. Return
+	 * early only if neither the size nor the allocation needs trimming.
+	 */
+	if (inode->i_size == HFS_I(inode)->phys_size && blk_cnt == alloc_cnt)
+		return;
+
 	if (blk_cnt == alloc_cnt)
 		goto out;
 
-- 
2.43.0