afs: Fix post-setattr file edit to do truncation correctly

At the end of an kAFS RPC operation, there is an "edit" phase (originally
intended for post-directory modification ops to edit the local image) that
the setattr VFS op uses to fix up the pagecache if the RPC that requested
truncation of a file was successful.

afs_setattr_edit_file() calls truncate_setsize() which sets i_size, expands
the pagecache if needed and truncates the pagecache.  The first two of
those, however, are redundant as they've already been done by
afs_setattr_success() under the io_lock and the first is also done under
the callback lock (cb_lock).

Fix afs_setattr_edit_file() to call truncate_pagecache() instead (which is
called by truncate_setsize(), thereby skipping the redundant parts.

Fixes: 100ccd18 ("netfs: Optimise away reads above the point at which there can be no data")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20240823200819.532106-3-dhowells@redhat.com
cc: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: Pankaj Raghav <p.raghav@samsung.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: netfs@lists.linux.dev
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/afs/inode.c

@@ -695,13 +695,18 @@ static void afs_setattr_edit_file(struct afs_operation *op)
 {
 	struct afs_vnode_param *vp = &op->file[0];
 	struct afs_vnode *vnode = vp->vnode;
+	struct inode *inode = &vnode->netfs.inode;
 
 	if (op->setattr.attr->ia_valid & ATTR_SIZE) {
 		loff_t size = op->setattr.attr->ia_size;
-		loff_t i_size = op->setattr.old_i_size;
+		loff_t old = op->setattr.old_i_size;
+
+		/* Note: inode->i_size was updated by afs_apply_status() inside
+		 * the I/O and callback locks.
+		 */
 
-		if (size != i_size) {
-			truncate_setsize(&vnode->netfs.inode, size);
+		if (size != old) {
+			truncate_pagecache(inode, size);
 			netfs_resize_file(&vnode->netfs, size, true);
 			fscache_resize_cookie(afs_vnode_cache(vnode), size);
 		}