netfs: Fix a few minor bugs in netfs_page_mkwrite()

We can't return with VM_FAULT_SIGBUS | VM_FAULT_LOCKED; the core
code will not unlock the folio in this instance.  Introduce a new
"unlock" error exit to handle this case.  Use it to handle
the "folio is truncated" check, and change the "writeback interrupted
by a fatal signal" to do a NOPAGE exit instead of letting the core
code install the folio currently under writeback before killing the
process.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241005182307.3190401-3-willy@infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/netfs/buffered_write.c

@@ -491,7 +491,9 @@ EXPORT_SYMBOL(netfs_file_write_iter);
 
 /*
  * Notification that a previously read-only page is about to become writable.
- * Note that the caller indicates a single page of a multipage folio.
+ * The caller indicates the precise page that needs to be written to, but
+ * we only track group on a per-folio basis, so we block more often than
+ * we might otherwise.
  */
 vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_group)
 {
@@ -501,7 +503,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 	struct address_space *mapping = file->f_mapping;
 	struct inode *inode = file_inode(file);
 	struct netfs_inode *ictx = netfs_inode(inode);
-	vm_fault_t ret = VM_FAULT_RETRY;
+	vm_fault_t ret = VM_FAULT_NOPAGE;
 	int err;
 
 	_enter("%lx", folio->index);
@@ -510,21 +512,15 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 
 	if (folio_lock_killable(folio) < 0)
 		goto out;
-	if (folio->mapping != mapping) {
-		folio_unlock(folio);
-		ret = VM_FAULT_NOPAGE;
-		goto out;
-	}
-
-	if (folio_wait_writeback_killable(folio)) {
-		ret = VM_FAULT_LOCKED;
-		goto out;
-	}
+	if (folio->mapping != mapping)
+		goto unlock;
+	if (folio_wait_writeback_killable(folio) < 0)
+		goto unlock;
 
 	/* Can we see a streaming write here? */
 	if (WARN_ON(!folio_test_uptodate(folio))) {
-		ret = VM_FAULT_SIGBUS | VM_FAULT_LOCKED;
-		goto out;
+		ret = VM_FAULT_SIGBUS;
+		goto unlock;
 	}
 
 	group = netfs_folio_group(folio);
@@ -559,5 +555,8 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 out:
 	sb_end_pagefault(inode->i_sb);
 	return ret;
+unlock:
+	folio_unlock(folio);
+	goto out;
 }
 EXPORT_SYMBOL(netfs_page_mkwrite);