[patch 3/7] memory-failure: fetch compound_head after pgmap_pfn_valid()

[Andrew Morton <akpm@linux-foundation.org>]

From: Joao Martins <joao.m.martins@oracle.com>
Subject: memory-failure: fetch compound_head after pgmap_pfn_valid()

memory_failure_dev_pagemap() at the moment assumes base pages (e.g. 
dax_lock_page()).  For devmap with compound pages fetch the compound_head
in case a tail page memory failure is being handled.

Currently this is a nop, but in the advent of compound pages in
dev_pagemap it allows memory_failure_dev_pagemap() to keep working.

Without this fix memory-failure handling (i.e.  MCEs on pmem) with
device-dax configured namespaces will regress (and crash).  

Link: https://lkml.kernel.org/r/20211202204422.26777-2-joao.m.martins@oracle.com
Reported-by: Jane Chu <jane.chu@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory-failure.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/mm/memory-failure.c~memory-failure-fetch-compound_head-after-pgmap_pfn_valid
+++ a/mm/memory-failure.c
@@ -1596,6 +1596,12 @@ static int memory_failure_dev_pagemap(un
 	}
 
 	/*
+	 * Pages instantiated by device-dax (not filesystem-dax)
+	 * may be compound pages.
+	 */
+	page = compound_head(page);
+
+	/*
 	 * Prevent the inode from being freed while we are interrogating
 	 * the address_space, typically this would be handled by
 	 * lock_page(), but dax pages do not use the page lock. This
_