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

[<gregkh@linuxfoundation.org>]

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>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From c93db7bb6ef3251e0ea48ade311d3e9942748e1c Mon Sep 17 00:00:00 2001
From: Matthew Wilcox <willy@infradead.org>
Date: Tue, 27 Nov 2018 13:16:33 -0800
Subject: [PATCH] dax: Check page->mapping isn't NULL

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>

diff --git a/fs/dax.c b/fs/dax.c
index 9bcce89ea18e..e69fc231833b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -365,7 +365,7 @@ bool dax_lock_mapping_entry(struct page *page)
 		struct address_space *mapping = READ_ONCE(page->mapping);
 
 		locked = false;
-		if (!dax_mapping(mapping))
+		if (!mapping || !dax_mapping(mapping))
 			break;
 
 		/*