[PATCH v2 1/2] mm/huge_memory: Adjust try_to_migrate_one() and split_huge_pmd_locked()

[Gavin Guo <gavinguo@igalia.com>]

The split_huge_pmd_locked function currently performs redundant checks
for migration entries and folio validation that are already handled by
the page_vma_mapped_walk mechanism in try_to_migrate_one.

Specifically, page_vma_mapped_walk already ensures that:
- The folio is properly mapped in the given VMA area
- pmd_trans_huge, pmd_devmap, and migration entry validation are
performed

To leverage page_vma_mapped_walk's work, moving TTU_SPLIT_HUGE_PMD
handling to the while loop checking and removing these duplicate checks
from split_huge_pmd_locked.

Suggested-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/all/98d1d195-7821-4627-b518-83103ade56c0@redhat.com/
Link: https://lore.kernel.org/all/91599a3c-e69e-4d79-bac5-5013c96203d7@redhat.com/
Signed-off-by: Gavin Guo <gavinguo@igalia.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c | 21 ++-------------------
 mm/rmap.c        | 18 +++++++++---------
 2 files changed, 11 insertions(+), 28 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 47d76d03ce30..485a0ba011af 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3075,27 +3075,10 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
 			   pmd_t *pmd, bool freeze, struct folio *folio)
 {
-	bool pmd_migration = is_pmd_migration_entry(*pmd);
-
-	VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
 	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
-	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
-	VM_BUG_ON(freeze && !folio);
-
-	/*
-	 * When the caller requests to set up a migration entry, we
-	 * require a folio to check the PMD against. Otherwise, there
-	 * is a risk of replacing the wrong folio.
-	 */
-	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
-		/*
-		 * Do not apply pmd_folio() to a migration entry; and folio lock
-		 * guarantees that it must be of the wrong folio anyway.
-		 */
-		if (folio && (pmd_migration || folio != pmd_folio(*pmd)))
-			return;
+	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
+	    is_pmd_migration_entry(*pmd))
 		__split_huge_pmd_locked(vma, pmd, address, freeze);
-	}
 }
 
 void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
diff --git a/mm/rmap.c b/mm/rmap.c
index 67bb273dfb80..b53a4dcaeaae 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2291,13 +2291,6 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 	if (flags & TTU_SYNC)
 		pvmw.flags = PVMW_SYNC;
 
-	/*
-	 * unmap_page() in mm/huge_memory.c is the only user of migration with
-	 * TTU_SPLIT_HUGE_PMD and it wants to freeze.
-	 */
-	if (flags & TTU_SPLIT_HUGE_PMD)
-		split_huge_pmd_address(vma, address, true, folio);
-
 	/*
 	 * For THP, we have to assume the worse case ie pmd for invalidation.
 	 * For hugetlb, it could be much worse if we need to do pud
@@ -2323,9 +2316,16 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 	mmu_notifier_invalidate_range_start(&range);
 
 	while (page_vma_mapped_walk(&pvmw)) {
-#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 		/* PMD-mapped THP migration entry */
 		if (!pvmw.pte) {
+			if (flags & TTU_SPLIT_HUGE_PMD) {
+				split_huge_pmd_locked(vma, pvmw.address,
+						      pvmw.pmd, true, NULL);
+				ret = false;
+				page_vma_mapped_walk_done(&pvmw);
+				break;
+			}
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 			subpage = folio_page(folio,
 				pmd_pfn(*pvmw.pmd) - folio_pfn(folio));
 			VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
@@ -2337,8 +2337,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				break;
 			}
 			continue;
-		}
 #endif
+		}
 
 		/* Unexpected PMD-mapped THP? */
 		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
-- 
2.43.0