From cb995f4eeba9d268fd4b56c2423ad6c1d1ea1b82 Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Fri, 17 Jun 2022 20:00:17 -0400
Subject: [PATCH] filemap: Handle sibling entries in filemap_get_read_batch()
Git-commit: cb995f4eeba9d268fd4b56c2423ad6c1d1ea1b82
Patch-mainline: v5.19-rc4
References: bsc#1202774

If a read races with an invalidation followed by another read, it is
possible for a folio to be replaced with a higher-order folio.  If that
happens, we'll see a sibling entry for the new folio in the next iteration
of the loop.  This manifests as a NULL pointer dereference while holding
the RCU read lock.

Handle this by simply returning.  The next call will find the new folio
and handle it correctly.  The other ways of handling this rare race are
more complex and it's just not worth it.

Reported-by: Dave Chinner <david@fromorbit.com>
Reported-by: Brian Foster <bfoster@redhat.com>
Debugged-by: Brian Foster <bfoster@redhat.com>
Tested-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Jan Kara <jack@suse.cz>

---
 mm/filemap.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2334,6 +2334,8 @@ static void filemap_get_read_batch(struc
 			continue;
 		if (xas.xa_index > max || xa_is_value(head))
 			break;
+		if (xa_is_sibling(head))
+			break;
 		if (!page_cache_get_speculative(head))
 			goto retry;