Right now we are inconsistent in our use of thp_migration_supported():

static inline bool thp_migration_supported(void)
{
	return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
}

And simply having arbitrary and ugly #ifdef
CONFIG_ARCH_ENABLE_THP_MIGRATION blocks in code.

This is exhibited in copy_huge_pmd(), which inserts a large #ifdef
CONFIG_ARCH_ENABLE_THP_MIGRATION block and an if-branch which is difficult
to follow

It's difficult to follow the logic of such a large function and the
non-present PMD logic is clearly separate as it sits in a giant if-branch.

Therefore this patch both separates out the logic and utilises
thp_migration_supported().

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/huge_memory.c | 109 +++++++++++++++++++++++++----------------------
 1 file changed, 59 insertions(+), 50 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2e5196a68f14..31116d69e289 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1774,6 +1774,62 @@ void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
 		update_mmu_cache_pmd(vma, addr, pmd);
 }
 
+static void copy_huge_non_present_pmd(
+		struct mm_struct *dst_mm, struct mm_struct *src_mm,
+		pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
+		struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
+		pmd_t pmd, pgtable_t pgtable)
+{
+	swp_entry_t entry = pmd_to_swp_entry(pmd);
+	struct folio *src_folio;
+
+	VM_WARN_ON(!is_pmd_non_present_folio_entry(pmd));
+
+	if (is_writable_migration_entry(entry) ||
+	    is_readable_exclusive_migration_entry(entry)) {
+		entry = make_readable_migration_entry(swp_offset(entry));
+		pmd = swp_entry_to_pmd(entry);
+		if (pmd_swp_soft_dirty(*src_pmd))
+			pmd = pmd_swp_mksoft_dirty(pmd);
+		if (pmd_swp_uffd_wp(*src_pmd))
+			pmd = pmd_swp_mkuffd_wp(pmd);
+		set_pmd_at(src_mm, addr, src_pmd, pmd);
+	} else if (is_device_private_entry(entry)) {
+		/*
+		 * For device private entries, since there are no
+		 * read exclusive entries, writable = !readable
+		 */
+		if (is_writable_device_private_entry(entry)) {
+			entry = make_readable_device_private_entry(swp_offset(entry));
+			pmd = swp_entry_to_pmd(entry);
+
+			if (pmd_swp_soft_dirty(*src_pmd))
+				pmd = pmd_swp_mksoft_dirty(pmd);
+			if (pmd_swp_uffd_wp(*src_pmd))
+				pmd = pmd_swp_mkuffd_wp(pmd);
+			set_pmd_at(src_mm, addr, src_pmd, pmd);
+		}
+
+		src_folio = pfn_swap_entry_folio(entry);
+		VM_WARN_ON(!folio_test_large(src_folio));
+
+		folio_get(src_folio);
+		/*
+		 * folio_try_dup_anon_rmap_pmd does not fail for
+		 * device private entries.
+		 */
+		folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page,
+					    dst_vma, src_vma);
+	}
+
+	add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+	mm_inc_nr_ptes(dst_mm);
+	pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
+	if (!userfaultfd_wp(dst_vma))
+		pmd = pmd_swp_clear_uffd_wp(pmd);
+	set_pmd_at(dst_mm, addr, dst_pmd, pmd);
+}
+
 int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		  pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
 		  struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
@@ -1819,59 +1875,12 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	ret = -EAGAIN;
 	pmd = *src_pmd;
 
-#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
-	if (unlikely(is_swap_pmd(pmd))) {
-		swp_entry_t entry = pmd_to_swp_entry(pmd);
-
-		VM_WARN_ON(!is_pmd_non_present_folio_entry(pmd));
-
-		if (is_writable_migration_entry(entry) ||
-		    is_readable_exclusive_migration_entry(entry)) {
-			entry = make_readable_migration_entry(swp_offset(entry));
-			pmd = swp_entry_to_pmd(entry);
-			if (pmd_swp_soft_dirty(*src_pmd))
-				pmd = pmd_swp_mksoft_dirty(pmd);
-			if (pmd_swp_uffd_wp(*src_pmd))
-				pmd = pmd_swp_mkuffd_wp(pmd);
-			set_pmd_at(src_mm, addr, src_pmd, pmd);
-		} else if (is_device_private_entry(entry)) {
-			/*
-			 * For device private entries, since there are no
-			 * read exclusive entries, writable = !readable
-			 */
-			if (is_writable_device_private_entry(entry)) {
-				entry = make_readable_device_private_entry(swp_offset(entry));
-				pmd = swp_entry_to_pmd(entry);
-
-				if (pmd_swp_soft_dirty(*src_pmd))
-					pmd = pmd_swp_mksoft_dirty(pmd);
-				if (pmd_swp_uffd_wp(*src_pmd))
-					pmd = pmd_swp_mkuffd_wp(pmd);
-				set_pmd_at(src_mm, addr, src_pmd, pmd);
-			}
-
-			src_folio = pfn_swap_entry_folio(entry);
-			VM_WARN_ON(!folio_test_large(src_folio));
-
-			folio_get(src_folio);
-			/*
-			 * folio_try_dup_anon_rmap_pmd does not fail for
-			 * device private entries.
-			 */
-			folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page,
-							dst_vma, src_vma);
-		}
-
-		add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
-		mm_inc_nr_ptes(dst_mm);
-		pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
-		if (!userfaultfd_wp(dst_vma))
-			pmd = pmd_swp_clear_uffd_wp(pmd);
-		set_pmd_at(dst_mm, addr, dst_pmd, pmd);
+	if (unlikely(thp_migration_supported() && is_swap_pmd(pmd))) {
+		copy_huge_non_present_pmd(dst_mm, src_mm, dst_pmd, src_pmd, addr,
+					  dst_vma, src_vma, pmd, pgtable);
 		ret = 0;
 		goto out_unlock;
 	}
-#endif
 
 	if (unlikely(!pmd_trans_huge(pmd))) {
 		pte_free(dst_mm, pgtable);
-- 
2.51.0