[PATCH 12/46] mm/huge_memory: fix livelock on device private folio

[David Hildenbrand (Arm)]

From: Josef Bacik <josef at toxicpanda.com>

If do_huge_pmd_device_private() cannot lock the folio, it simply returns
to try again on the next fault.

Just like we did in commit b570f37a2ce4 ("mm: Fix a hmm_range_fault()
livelock / starvation problem"), let's wait for the softleaf entry,
which will drop the PTL and wait until the folio was unlocked. This is
precisely what we do in do_swap_page() with device-private folios. This
avoids possibly livelocking.

Update softleaf_entry_wait_on_locked() documentation to clarify that
device-private entries wait until the folio is unlocked.

There seems to be no compelling reason to backport this to stable
kernels unless we actually see problems there.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 4964099163d0 ("mm/memory/fault: add THP fault handling for zone device private pages")
Signed-off-by: Josef Bacik <josef at toxicpanda.com>
[ david: simplify patch description, fixup softleaf_entry_wait_on_locked description]
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/filemap.c     | 4 ++--
 mm/huge_memory.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b04be64a5069f..e85a4114d2ecc 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1395,8 +1395,8 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
 
 #ifdef CONFIG_MIGRATION
 /**
- * softleaf_entry_wait_on_locked - Wait for a migration entry or
- * device_private entry to be removed.
+ * softleaf_entry_wait_on_locked - Wait for a migration entry to be removed
+ * or a device-private folio in a device-private entry to get unlocked.
  * @entry: migration or device_private swap entry.
  * @ptl: already locked ptl. This function will drop the lock.
  *
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 283c4bc8b927c..2b31a425c7a32 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1500,7 +1500,7 @@ vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
 		folio_unlock(folio);
 		folio_put(folio);
 	} else {
-		spin_unlock(ptl);
+		softleaf_entry_wait_on_locked(entry, ptl);
 	}
 
 	return ret;

-- 
2.43.0