[PATCH v3 11/16] mm: introduce pmd_is_huge() and use where appropriate

Lorenzo Stoakes
2025-11-10T22:24:15Z

The leaf entry PMD case is confusing as only migration entries and
device private entries are valid at PMD level, not true swap entries.

We repeatedly perform checks of the form is_swap_pmd() || pmd_trans_huge()
which is itself confusing - it implies that leaf entries at PMD level exist
and are different from huge entries.

Address this confusion by introduced pmd_is_huge() which checks for either
case. Sadly due to header dependency issues (huge_mm.h is included very
early on in headers and cannot really rely on much else) we cannot use
pmd_is_valid_softleaf() here.

However since these are the only valid, handled cases the function is still
achieving what it intends to do.

We then replace all instances of is_swap_pmd() || pmd_trans_huge() with
pmd_is_huge() invocations and adjust logic accordingly to accommodate
this.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
include/linux/huge_mm.h | 39 +++++++++++++++++++++++++++++++++++----
include/linux/swapops.h | 6 ++++++
mm/huge_memory.c | 3 ++-
mm/memory.c | 4 ++--
mm/mprotect.c | 2 +-
mm/mremap.c | 2 +-
6 files changed, 47 insertions(+), 9 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index cbb2243f8e56..403e13009631 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -419,10 +419,36 @@ void reparent_deferred_split_queue(struct mem_cgroup *memcg);
void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
unsigned long address, bool freeze);

+/**
+ * pmd_is_huge() - Is this PMD either a huge PMD entry or a software leaf entry?
+ * @pmd: The PMD to check.
+ *
+ * A huge PMD entry is a non-empty entry which is present and marked huge or a
+ * software leaf entry. This check be performed without the appropriate locks
+ * held, in which case the condition should be rechecked after they are
+ * acquired.
+ *
+ * Returns: true if this PMD is huge, false otherwise.
+ */
+static inline bool pmd_is_huge(pmd_t pmd)
+{
+ if (pmd_present(pmd)) {
+ return pmd_trans_huge(pmd);
+ } else if (!pmd_none(pmd)) {
+ /*
+ * Non-present PMDs must be valid huge non-present entries. We
+ * cannot assert that here due to header dependency issues.
+ */
+ return true;
+ }
+
+ return false;
+}
+
#define split_huge_pmd(__vma, __pmd, __address) \
do { \
pmd_t *____pmd = (__pmd); \
- if (is_swap_pmd(*____pmd) || pmd_trans_huge(*____pmd)) \
+ if (pmd_is_huge(*____pmd)) \
__split_huge_pmd(__vma, __pmd, __address, \
false); \
} while (0)
@@ -469,10 +495,10 @@ static inline int is_swap_pmd(pmd_t pmd)
static inline spinlock_t *pmd_trans_huge_lock(pmd_t *pmd,
struct vm_area_struct *vma)
{
- if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd))
+ if (pmd_is_huge(*pmd))
return __pmd_trans_huge_lock(pmd, vma);
- else
- return NULL;
+
+ return NULL;
}
static inline spinlock_t *pud_trans_huge_lock(pud_t *pud,
struct vm_area_struct *vma)
@@ -744,6 +770,11 @@ static inline struct folio *get_persistent_huge_zero_folio(void)
{
return NULL;
}
+
+static inline bool pmd_is_huge(pmd_t pmd)
+{
+ return false;
+}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

static inline int split_folio_to_list_to_order(struct folio *folio,
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index f1277647262d..41cfc6d59054 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -471,6 +471,12 @@ static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
}

#else /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
+static inline int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
+ struct page *page)
+{
+ BUILD_BUG();
+}
+
static inline void remove_migration_pmd(struct page_vma_mapped_walk *pvmw,
struct page *new)
{
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5876595b00d5..2f0bdc987596 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2810,8 +2810,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
{
spinlock_t *ptl;
+
ptl = pmd_lock(vma->vm_mm, pmd);
- if (likely(is_swap_pmd(*pmd) || pmd_trans_huge(*pmd)))
+ if (likely(pmd_is_huge(*pmd)))
return ptl;
spin_unlock(ptl);
return NULL;
diff --git a/mm/memory.c b/mm/memory.c
index bf2bbd0dbc97..087f31a291b4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1374,7 +1374,7 @@ copy_pmd_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
src_pmd = pmd_offset(src_pud, addr);
do {
next = pmd_addr_end(addr, end);
- if (is_swap_pmd(*src_pmd) || pmd_trans_huge(*src_pmd)) {
+ if (pmd_is_huge(*src_pmd)) {
int err;

VM_BUG_ON_VMA(next-addr != HPAGE_PMD_SIZE, src_vma);
@@ -1923,7 +1923,7 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
pmd = pmd_offset(pud, addr);
do {
next = pmd_addr_end(addr, end);
- if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd)) {
+ if (pmd_is_huge(*pmd)) {
if (next - addr != HPAGE_PMD_SIZE)
__split_huge_pmd(vma, pmd, addr, false);
else if (zap_huge_pmd(tlb, vma, pmd, addr)) {
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a3e360a8cdec..ab014ce17f9c 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -474,7 +474,7 @@ static inline long change_pmd_range(struct mmu_gather *tlb,
goto next;

_pmd = pmdp_get_lockless(pmd);
- if (is_swap_pmd(_pmd) || pmd_trans_huge(_pmd)) {
+ if (pmd_is_huge(_pmd)) {
if ((next - addr != HPAGE_PMD_SIZE) ||
pgtable_split_needed(vma, cp_flags)) {
__split_huge_pmd(vma, pmd, addr, false);
diff --git a/mm/mremap.c b/mm/mremap.c
index 62b6827abacf..fdb0485ede74 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -850,7 +850,7 @@ unsigned long move_page_tables(struct pagetable_move_control *pmc)
if (!new_pmd)
break;
again:
- if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd)) {
+ if (pmd_is_huge(*old_pmd)) {
if (extent == HPAGE_PMD_SIZE &&
move_pgt_entry(pmc, HPAGE_PMD, old_pmd, new_pmd))
continue;
--
2.51.0