FAILED: patch "[PATCH] mm/huge_memory: fix folio isn't locked in softleaf_to_folio()" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-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 <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 4ff07459db888054f68575646d7fe04f31f1e56d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050331-napped-catnap-947d@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 4ff07459db888054f68575646d7fe04f31f1e56d Mon Sep 17 00:00:00 2001
From: Jinjiang Tu <[email protected]>
Date: Thu, 19 Mar 2026 09:25:41 +0800
Subject: [PATCH] mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

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().

index a9ff94b744f2..05673d3529e7 100644
@@ -363,6 +363,23 @@ static inline unsigned long softleaf_to_pfn(softleaf_t entry)
 	return swp_offset(entry) & SWP_PFN_MASK;
 }
 
+static inline void softleaf_migration_sync(softleaf_t entry,
+		struct folio *folio)
+{
+	/*
+	 * 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
+	 */
+	VM_WARN_ON_ONCE(!folio_test_locked(folio));
+}
+
 /**
  * softleaf_to_page() - Obtains struct page for PFN encoded within leaf entry.
  * @entry: Leaf entry, softleaf_has_pfn(@entry) must return true.

@@ -374,11 +391,8 @@ static inline struct page *softleaf_to_page(softleaf_t entry)
 	struct page *page = pfn_to_page(softleaf_to_pfn(entry));
 
 	VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
-	/*
-	 * Any use of migration entries may only occur while the
-	 * corresponding page is locked
-	 */
-	VM_WARN_ON_ONCE(softleaf_is_migration(entry) && !PageLocked(page));
+	if (softleaf_is_migration(entry))
+		softleaf_migration_sync(entry, page_folio(page));
 
 	return page;
 }

@@ -394,12 +408,8 @@ static inline struct folio *softleaf_to_folio(softleaf_t entry)
 	struct folio *folio = pfn_folio(softleaf_to_pfn(entry));
 
 	VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
-	/*
-	 * Any use of migration entries may only occur while the
-	 * corresponding folio is locked.
-	 */
-	VM_WARN_ON_ONCE(softleaf_is_migration(entry) &&
-			!folio_test_locked(folio));
+	if (softleaf_is_migration(entry))
+		softleaf_migration_sync(entry, folio);
 
 	return folio;
 }