[PATCH v2] mm/memory-failure: fix infinite UCE for VM_PFNMAP'ed page

[Jinjiang Tu <tujinjiang@huawei.com>]

When memory_failure() is called for a already hwpoisoned pfn backed with
struct page, kill_accessing_process() will conditionally send a SIGBUS to
the current (triggering) process if it maps the page.

However, in case the page is not ordinarily mapped, but was mapped through
remap_pfn_range(), kill_accessing_process() wouldn't identify it as mapped
even though hwpoison_pte_range() would be prepared to handle it, because
walk_page_range() will skip VM_PFNMAP as default in walk_page_test(). As
a result, walk_page_range() will return 0, assuming "not mapped" and SIGBUS
will be skipped. The user task will trigger UCE infinitely because it will
not receive a SIGBUS on access and simply retry.

Before commit aaf99ac2ceb7 ("mm/hwpoison: do not send SIGBUS to processes
with recovered clean pages"), kill_accessing_process() will return EFAULT.
For x86, the current task will be killed in kill_me_maybe().

To fix it, add .test_walk callback for hwpoison_walk_ops to process
VM_PFNMAP VMAs too.

Fixes: aaf99ac2ceb7 ("mm/hwpoison: do not send SIGBUS to processes with recovered clean pages")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
---
Changelog since v1:
 * update patch description, suggested by David Hildenbrand 

 mm/memory-failure.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index e2e685b971bb..fa6a8f2cdebc 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -853,9 +853,16 @@ static int hwpoison_hugetlb_range(pte_t *ptep, unsigned long hmask,
 #define hwpoison_hugetlb_range	NULL
 #endif
 
+static int hwpoison_test_walk(unsigned long start, unsigned long end,
+			     struct mm_walk *walk)
+{
+	return 0;
+}
+
 static const struct mm_walk_ops hwpoison_walk_ops = {
 	.pmd_entry = hwpoison_pte_range,
 	.hugetlb_entry = hwpoison_hugetlb_range,
+	.test_walk = hwpoison_test_walk,
 	.walk_lock = PGWALK_RDLOCK,
 };
 
-- 
2.34.1