[PATCH v2 5/5] smb: client: discard post-EOF pagecache when extending a file via clone range

[Paulo Alcantara <pc@manguebit.org>]

cifs_remap_file_range() discarded the pagecache only for the folios
overlapping the destination region, so when a clone extends the file the
folio straddling the old EOF - which may hold data dirtied past EOF
through an mmap - was never dropped and became visible as file content
once the file was extended.

Start the discard at the smaller of fstart and the old EOF.  i_size is
still old here, so nothing past the old EOF is flushed to the server.

Fixes: c54fc3a4f375 ("cifs: Fix flushing, invalidation and file size with FICLONE")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
---
 fs/smb/client/cifsfs.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index f1fb4b7b7b7c..6410249f529a 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -1467,9 +1467,14 @@ static loff_t cifs_remap_file_range(struct file *src_file, loff_t off,
 	i_size = target_inode->i_size;
 	spin_unlock(&target_inode->i_lock);
 
-	/* Discard all the folios that overlap the destination region. */
+	/*
+	 * Discard all the folios that overlap the destination region.  Start at
+	 * the old EOF when extending so the folio straddling it, which may hold
+	 * data written past EOF through an mmap, is dropped too.
+	 */
 	cifs_dbg(FYI, "about to discard pages %llx-%llx\n", fstart, fend);
-	truncate_inode_pages_range(&target_inode->i_data, fstart, fend);
+	truncate_inode_pages_range(&target_inode->i_data,
+				   min(fstart, i_size), fend);
 
 	fscache_invalidate(cifs_inode_cookie(target_inode), NULL, i_size, 0);
 
-- 
2.55.0