Re: FAILED: patch "[PATCH] dax: Check page->mapping isn't NULL" failed to apply to 4.19-stable tree

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 11, 2018 at 03:00:15PM +0100, gregkh@linuxfoundation.org wrote:
> The patch below does not apply to the 4.19-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.


>From ebcfdf7933e1c72dbe581e64064bc6b8bb5d8694 Mon Sep 17 00:00:00 2001
From: Matthew Wilcox <willy@infradead.org>
Date: Tue, 11 Dec 2018 09:41:48 -0500
Subject: [PATCH 1/2] dax: Check page->mapping isn't NULL

commit c93db7bb6ef3251e0ea48ade311d3e9942748e1c upstream.

If we race with inode destroy, it's possible for page->mapping to be
NULL before we even enter this routine, as well as after having slept
waiting for the dax entry to become unlocked.

Fixes: c2a7d2a11552 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Cc: <stable@vger.kernel.org>
Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 fs/dax.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index b0cd1364c68f..3a2682a6c832 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -423,7 +423,7 @@ bool dax_lock_mapping_entry(struct page *page)
 	for (;;) {
 		mapping = READ_ONCE(page->mapping);
 
-		if (!dax_mapping(mapping))
+		if (!mapping || !dax_mapping(mapping))
 			break;
 
 		/*
-- 
2.19.1