afs: Fix afs_write_end() to handle short writes

Fix afs_write_end() to correctly handle a short copy into the intended
write region of the page.  Two things are necessary:

 (1) If the page is not up to date, then we should just return 0
     (ie. indicating a zero-length copy).  The loop in
     generic_perform_write() will go around again, possibly breaking up the
     iterator into discrete chunks[1].

     This is analogous to commit b9de313
     for ceph.

 (2) The page should not have been set uptodate if it wasn't completely set
     up by netfs_write_begin() (this will be fixed in the next patch), so
     we need to set uptodate here in such a case.

Also remove the assertion that was checking that the page was set uptodate
since it's now set uptodate if it wasn't already a few lines above.  The
assertion was from when uptodate was set elsewhere.

Changes:
v3: Remove the handling of len exceeding the end of the page.

Fixes: 3003bbd ("afs: Use the netfs_write_begin() helper")
Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/YMwVp268KTzTf8cN@zeniv-ca.linux.org.uk/ [1]
Link: https://lore.kernel.org/r/162367682522.460125.5652091227576721609.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/162391825688.1173366.3437507255136307904.stgit@warthog.procyon.org.uk/ # v2

Author: dhowells

fs/afs/write.c

@@ -118,6 +118,15 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 	_enter("{%llx:%llu},{%lx}",
 	       vnode->fid.vid, vnode->fid.vnode, page->index);
 
+	if (!PageUptodate(page)) {
+		if (copied < len) {
+			copied = 0;
+			goto out;
+		}
+
+		SetPageUptodate(page);
+	}
+
 	if (copied == 0)
 		goto out;
 
@@ -132,8 +141,6 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 		write_sequnlock(&vnode->cb_lock);
 	}
 
-	ASSERT(PageUptodate(page));
-
 	if (PagePrivate(page)) {
 		priv = page_private(page);
 		f = afs_page_dirty_from(page, priv);