[PATCH 6.1.y] mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

[Sasha Levin <sashal@kernel.org>]

From: Jinjiang Tu <tujinjiang@huawei.com>

[ Upstream commit 4c5e7f0fcd592801c9cc18f29f80fbee84eb8669 ]

On arm64 server, we found folio that get from migration entry isn't locked
in softleaf_to_folio().  This issue triggers when mTHP splitting and
zap_nonpresent_ptes() races, and the root cause is lack of memory barrier
in softleaf_to_folio().  The race is as follows:

	CPU0                                             CPU1

deferred_split_scan()                              zap_nonpresent_ptes()
  lock folio
  split_folio()
    unmap_folio()
      change ptes to migration entries
    __split_folio_to_order()                         softleaf_to_folio()
      set flags(including PG_locked) for tail pages    folio = pfn_folio(softleaf_to_pfn(entry))
      smp_wmb()                                        VM_WARN_ON_ONCE(!folio_test_locked(folio))
      prep_compound_page() for tail pages

In __split_folio_to_order(), smp_wmb() guarantees page flags of tail pages
are visible before the tail page becomes non-compound.  smp_wmb() should
be paired with smp_rmb() in softleaf_to_folio(), which is missed.  As a
result, if zap_nonpresent_ptes() accesses migration entry that stores tail
pfn, softleaf_to_folio() may see the updated compound_head of tail page
before page->flags.

This issue will trigger VM_WARN_ON_ONCE() in pfn_swap_entry_folio()
because of the race between folio split and zap_nonpresent_ptes()
leading to a folio incorrectly undergoing modification without a folio
lock being held.

This is a BUG_ON() before commit 93976a20345b ("mm: eliminate further
swapops predicates"), which in merged in v6.19-rc1.

To fix it, add missing smp_rmb() if the softleaf entry is migration entry
in softleaf_to_folio() and softleaf_to_page().

[tujinjiang@huawei.com: update function name and comments]
  Link: https://lkml.kernel.org/r/20260321075214.3305564-1-tujinjiang@huawei.com
Link: https://lkml.kernel.org/r/20260319012541.4158561-1-tujinjiang@huawei.com
Fixes: e9b61f19858a ("thp: reintroduce split_huge_page()")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ adapted fix from leafops.h softleaf_to_page()/softleaf_to_folio() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/swapops.h | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 1f59f9edcc241..7de89a03b72e7 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -541,11 +541,21 @@ static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
 {
 	struct page *p = pfn_to_page(swp_offset_pfn(entry));
 
-	/*
-	 * Any use of migration entries may only occur while the
-	 * corresponding page is locked
-	 */
-	BUG_ON(is_migration_entry(entry) && !PageLocked(p));
+	if (is_migration_entry(entry)) {
+		/*
+		 * Ensure we do not race with split, which might alter tail
+		 * pages into new folios and thus result in observing an
+		 * unlocked folio.
+		 * This matches the write barrier in __split_folio_to_order().
+		 */
+		smp_rmb();
+
+		/*
+		 * Any use of migration entries may only occur while the
+		 * corresponding page is locked
+		 */
+		BUG_ON(!PageLocked(p));
+	}
 
 	return p;
 }
-- 
2.53.0