[PATCH 5/5] exfat: add support for SEEK_HOLE and SEEK_DATA in llseek

[Namjae Jeon <linkinjeon@kernel.org>]

Adds exfat_file_llseek() that implements these whence values via
the iomap layer (iomap_seek_hole() and iomap_seek_data()) using the
existing exfat_read_iomap_ops.
Unlike many other modern filesystems, exFAT does not support sparse files
with unallocated clusters (holes). In exFAT, clusters are always fully
allocated once they are written or preallocated. In addition, exFAT
maintains a separate "Valid Data Length" (valid_size) that is distinct
from the logical file size. This affects how holes are reported during
seeking. In exfat_read_iomap_begin(), ranges where the offset is greater
than or equal to ei->valid_size are mapped as IOMAP_UNWRITTEN, while ranges
below valid_size are mapped as IOMAP_MAPPED. This mapping behavior is used
by the iomap seek functions to correctly report SEEK_HOLE and SEEK_DATA
positions.

  - Ranges with offset >= ei->valid_size are mapped as IOMAP_UNWRITTEN.
  - Ranges with offset < ei->valid_size are mapped as IOMAP_MAPPED.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
---
 fs/exfat/file.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index d7857aec072b..9a30c32b3a05 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -884,9 +884,34 @@ static int exfat_file_open(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+static loff_t exfat_file_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct inode *inode = file->f_mapping->host;
+
+	switch (whence) {
+	case SEEK_HOLE:
+		inode_lock_shared(inode);
+		offset = iomap_seek_hole(inode, offset, &exfat_read_iomap_ops);
+		inode_unlock_shared(inode);
+		break;
+	case SEEK_DATA:
+		inode_lock_shared(inode);
+		offset = iomap_seek_data(inode, offset, &exfat_read_iomap_ops);
+		inode_unlock_shared(inode);
+		break;
+	default:
+		return generic_file_llseek_size(file, offset, whence,
+						inode->i_sb->s_maxbytes,
+						i_size_read(inode));
+	}
+	if (offset < 0)
+		return offset;
+	return vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
+}
+
 const struct file_operations exfat_file_operations = {
 	.open		= exfat_file_open,
-	.llseek		= generic_file_llseek,
+	.llseek		= exfat_file_llseek,
 	.read_iter	= exfat_file_read_iter,
 	.write_iter	= exfat_file_write_iter,
 	.unlocked_ioctl = exfat_ioctl,
-- 
2.25.1