mm: validate inode in mapping_set_error()

The swap address_space doesn't have host. Thus, it makes kernel crash once
swap write meets error. Fix it.

Fixes: 735e4ae ("vfs: track per-sb writeback errors and report them to syncfs")
Signed-off-by: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Jeff Layton <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: David Howells <[email protected]>
Cc: <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: minchank

include/linux/pagemap.h

@@ -54,7 +54,8 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
 	__filemap_set_wb_err(mapping, error);
 
 	/* Record it in superblock */
-	errseq_set(&mapping->host->i_sb->s_wb_err, error);
+	if (mapping->host)
+		errseq_set(&mapping->host->i_sb->s_wb_err, error);
 
 	/* Record it in flags for now, for legacy callers */
 	if (error == -ENOSPC)