[PATCH v2 9/9] smb/client: emulate small sparse fallocate ranges at EOF

[Huiwen He <huiwen.he@linux.dev>]

From: Huiwen He <hehuiwen@kylinos.cn>

smb3_simple_falloc() returns -EOPNOTSUPP for mode 0 requests that start
at EOF of a non-empty sparse file because FILE_ALLOCATION_INFORMATION
cannot allocate an arbitrary byte range.

However, one small case can be handled safely: a range that starts exactly
at the current EOF. For example:

        xfs_io -f -c "pwrite 0 1m" file
        xfs_io -c "falloc 1m 4k" file

Writing zeroes to this range does not create an intervening hole or
overwrite existing data. It allocates the requested range and extends EOF
without changing other sparse regions. Limit this emulation to 1 MiB to
keep the network and server I/O cost bounded.

This allows small EOF-adjacent fallocate requests on sparse files to
succeed. After the write, update the local file size and refresh i_blocks
from the server-reported AllocationSize. Larger ranges remain
unsupported.

Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
---
 fs/smb/client/smb2ops.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/fs/smb/client/smb2ops.c b/fs/smb/client/smb2ops.c
index e0815805caca..b6d5771a7c54 100644
--- a/fs/smb/client/smb2ops.c
+++ b/fs/smb/client/smb2ops.c
@@ -3707,6 +3707,44 @@ static long smb3_simple_falloc(struct file *file, struct cifs_tcon *tcon,
 		if (rc)
 			goto out;
 
+		/*
+		 * A small range immediately after EOF can be allocated by
+		 * writing zeroes without creating an intervening hole.
+		 */
+		if (off == old_eof && old_eof != 0 &&
+		    (cifsi->cifsAttrs & FILE_ATTRIBUTE_SPARSE_FILE)) {
+			if (len > 1024 * 1024) {
+				rc = -EOPNOTSUPP;
+				goto out;
+			}
+
+			rc = smb3_simple_fallocate_range(xid, tcon, cfile,
+							 off, len);
+			if (rc) {
+				spin_lock(&inode->i_lock);
+				cifsi->time = 0;
+				spin_unlock(&inode->i_lock);
+				goto out;
+			}
+
+			new_eof = off + len;
+			netfs_resize_file(&cifsi->netfs, new_eof, true);
+			cifs_setsize(inode, new_eof);
+
+			qrc = SMB2_query_info(xid, tcon,
+					      cfile->fid.persistent_fid,
+					      cfile->fid.volatile_fid, &file_inf);
+			spin_lock(&inode->i_lock);
+			if (qrc == 0) {
+				asize = le64_to_cpu(file_inf.AllocationSize);
+				inode->i_blocks = CIFS_INO_BLOCKS(asize);
+			} else {
+				cifsi->time = 0;
+			}
+			spin_unlock(&inode->i_lock);
+			goto out;
+		}
+
 		/*
 		 * FILE_ALLOCATION_INFORMATION sets the allocation size for the whole
 		 * file and cannot allocate an arbitrary byte range. Use it only for
-- 
2.43.0