[PATCH v6 03/12] mm: add PMD swap entry splitting support

[Usama Arif <usama.arif@linux.dev>]

Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap
entry into 512 PTE swap entries. No folio reference is needed because
swap entries point to swap slots rather than pages. Each PTE inherits
the correct sub-slot offset and preserves soft_dirty, uffd_wp, and
exclusive flags.

The folio_remove_rmap_pmd() gate at the end must inspect old_pmd
rather than *pmd: for a present THP split, *pmd has already been
cleared by pmdp_invalidate(), and that invalidated bit pattern can
decode as a plausible swap entry.

This branch is reached from the explicit __split_huge_pmd() callers
that hit a non-present PMD: partial-range mprotect / munmap, the
wp_huge_pmd() PMD-COW fallback, and the swap-in / swapoff fallbacks
added in later patches when the cached folio is no longer PMD-sized.
page_vma_mapped_walk() does not iterate PMD swap entries, so
try_to_unmap_one() and try_to_migrate_one() do not reach this branch
and freeze=true cannot occur in this branch today.  page and folio
are therefore left uninitialized in the swap branch; a
VM_WARN_ON_ONCE(freeze) catches any future caller that breaks this
invariant before the freeze path dereferences page_to_pfn(page + i)
or put_page(page).

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/huge_memory.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1b6b0aa2baa3b..a473e85d30f51 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3252,6 +3252,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
 						 vma, haddr, rmap_flags);
 		}
+	} else if (pmd_is_swap_entry(*pmd)) {
+		VM_WARN_ON_ONCE(freeze);
+		/* Swap entries have no page for the migration freeze path. */
+		freeze = false;
+		old_pmd = *pmd;
+		soft_dirty = pmd_swp_soft_dirty(old_pmd);
+		uffd_wp = pmd_swp_uffd(old_pmd);
+		anon_exclusive = pmd_swp_exclusive(old_pmd);
 	} else {
 		/*
 		 * Up to this point the pmd is present and huge and userland has
@@ -3388,6 +3396,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
 			set_pte_at(mm, addr, pte + i, entry);
 		}
+	} else if (pmd_is_swap_entry(old_pmd)) {
+		softleaf_t sl_entry = softleaf_from_pmd(old_pmd);
+		pte_t swp_pte;
+		swp_entry_t sub_entry;
+
+		for (i = 0, addr = haddr; i < HPAGE_PMD_NR;
+		     i++, addr += PAGE_SIZE) {
+			sub_entry = swp_entry(swp_type(sl_entry),
+					      swp_offset(sl_entry) + i);
+			swp_pte = swp_entry_to_pte(sub_entry);
+			if (soft_dirty)
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			if (uffd_wp)
+				swp_pte = pte_swp_mkuffd(swp_pte);
+			if (anon_exclusive)
+				swp_pte = pte_swp_mkexclusive(swp_pte);
+			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+			set_pte_at(mm, addr, pte + i, swp_pte);
+		}
 	} else {
 		pte_t entry;
 
@@ -3415,7 +3442,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	}
 	pte_unmap(pte);
 
-	if (!pmd_is_migration_entry(*pmd))
+	if (!pmd_is_migration_entry(old_pmd) && !pmd_is_swap_entry(old_pmd))
 		folio_remove_rmap_pmd(folio, page, vma);
 	if (freeze)
 		put_page(page);
-- 
2.53.0-Meta