[PATCH v4] mm/vmscan: fix hwpoisoned large folio handling in shrink_folio_list

[Jinjiang Tu <tujinjiang@huawei.com>]

In shrink_folio_list(), the hwpoisoned folio may be large folio, which
can't be handled by unmap_poisoned_folio(). For THP, try_to_unmap_one()
must be passed with TTU_SPLIT_HUGE_PMD to split huge PMD first and then
retry. Without TTU_SPLIT_HUGE_PMD, we will trigger null-ptr deref of
pvmw.pte. Even we passed TTU_SPLIT_HUGE_PMD, we will trigger a WARN_ON_ONCE
due to the page isn't in swapcache.

Since UCE is rare in real world, and race with reclaimation is more rare,
just skipping the hwpoisoned large folio is enough. memory_failure() will
handle it if the UCE is triggered again.

Fixes: 1b0449544c64 ("mm/vmscan: don't try to reclaim hwpoison folio")
Reported-by: syzbot+3b220254df55d8ca8a61@syzkaller.appspotmail.com
Closes: 
https://lore.kernel.org/all/68412d57.050a0220.2461cf.000e.GAE@google.com/
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
---
  mm/memory-failure.c | 4 ++++
  mm/vmscan.c         | 8 ++++++++
  2 files changed, 12 insertions(+)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index b91a33fb6c69..225dddff091d 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1561,6 +1561,10 @@ static int get_hwpoison_page(struct page *p, 
unsigned long flags)
      return ret;
  }

+/*
+ * The caller must guarantee the folio isn't large folio, except hugetlb.
+ * try_to_unmap() can't handle it.
+ */
  int unmap_poisoned_folio(struct folio *folio, unsigned long pfn, bool 
must_kill)
  {
      enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_SYNC | TTU_HWPOISON;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f8dfd2864bbf..424412680cfc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1138,6 +1138,14 @@ static unsigned int shrink_folio_list(struct 
list_head *folio_list,
              goto keep;

          if (folio_contain_hwpoisoned_page(folio)) {
+            /*
+             * unmap_poisoned_folio() can't handle large
+             * folio, just skip it. memory_failure() will
+             * handle it if the UCE is triggered again.
+             */
+            if (folio_test_large(folio))
+                goto keep_locked;
+
              unmap_poisoned_folio(folio, folio_pfn(folio), false);
              folio_unlock(folio);
              folio_put(folio);
-- 
2.43.0