[RFC PATCH 03/12] mm: introduce get_pte_swap_entry() and use it

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

We have a number of checks which explicitly check for 'true' swap entries,
that is swap entries which are not non-swap swap entries.

This is a confusing state of affairs, and we're duplicating checks as well
as using is_swap_pte() which is applied inconsistently throughout the code
base.

Avoid all this by introducing a new function, get_pte_swap_entry() that
explicitly checks for a true swap entry and returns it if the PTE contains
one.

We then update the code base to use this function.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/swapops.h | 29 +++++++++++++++++++++++++++++
 mm/internal.h           |  6 +++---
 mm/madvise.c            |  5 +----
 mm/swap_state.c         |  5 +----
 mm/swapfile.c           |  3 +--
 5 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 24eaf8825c6b..a557b0e7f05c 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -649,5 +649,34 @@ static inline int is_pmd_non_present_folio_entry(pmd_t pmd)
 	return is_pmd_migration_entry(pmd) || is_pmd_device_private_entry(pmd);
 }
 
+/**
+ * get_pte_swap_entry() - Gets PTE swap entry if one is present.
+ * @pte: The PTE we are checking.
+ * @entryp: Output pointer to a swap entry that will be populated upon
+ * success.
+ *
+ * Determines if the PTE describes an entry in swap or swap cache (i.e. is a
+ * swap entry and not a non-swap entry), if so it sets @entryp to the swap
+ * entry.
+ *
+ * This should only be used if we do not have any prior knowledge of this
+ * PTE's state.
+ *
+ * Return: true if swappable, false otherwise.
+ */
+static inline bool get_pte_swap_entry(pte_t pte, swp_entry_t *entryp)
+{
+	if (pte_present(pte))
+		return false;
+	if (pte_none(pte))
+		return false;
+
+	*entryp = pte_to_swp_entry(pte);
+	if (non_swap_entry(*entryp))
+		return false;
+
+	return true;
+}
+
 #endif /* CONFIG_MMU */
 #endif /* _LINUX_SWAPOPS_H */
diff --git a/mm/internal.h b/mm/internal.h
index b855a4412878..78dcf6048672 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -378,15 +378,15 @@ static inline pte_t pte_next_swp_offset(pte_t pte)
  */
 static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
 {
+	swp_entry_t entry;
+	const bool __maybe_unused is_swap = get_pte_swap_entry(pte, &entry);
 	pte_t expected_pte = pte_next_swp_offset(pte);
 	const pte_t *end_ptep = start_ptep + max_nr;
-	swp_entry_t entry = pte_to_swp_entry(pte);
 	pte_t *ptep = start_ptep + 1;
 	unsigned short cgroup_id;
 
 	VM_WARN_ON(max_nr < 1);
-	VM_WARN_ON(!is_swap_pte(pte));
-	VM_WARN_ON(non_swap_entry(entry));
+	VM_WARN_ON(!is_swap);
 
 	cgroup_id = lookup_swap_cgroup_id(entry);
 	while (ptep < end_ptep) {
diff --git a/mm/madvise.c b/mm/madvise.c
index f9f80b2e9d43..578036ef6675 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -205,10 +205,7 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,
 		}
 
 		pte = ptep_get(ptep);
-		if (!is_swap_pte(pte))
-			continue;
-		entry = pte_to_swp_entry(pte);
-		if (unlikely(non_swap_entry(entry)))
+		if (!get_pte_swap_entry(pte, &entry))
 			continue;
 
 		pte_unmap_unlock(ptep, ptl);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index b13e9c4baa90..9199b64206ff 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -754,10 +754,7 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 				break;
 		}
 		pentry = ptep_get_lockless(pte);
-		if (!is_swap_pte(pentry))
-			continue;
-		entry = pte_to_swp_entry(pentry);
-		if (unlikely(non_swap_entry(entry)))
+		if (!get_pte_swap_entry(pentry, &entry))
 			continue;
 		pte_unmap(pte);
 		pte = NULL;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index cb2392ed8e0e..74eb9221a220 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2253,10 +2253,9 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 
 		ptent = ptep_get_lockless(pte);
 
-		if (!is_swap_pte(ptent))
+		if (!get_pte_swap_entry(ptent, &entry))
 			continue;
 
-		entry = pte_to_swp_entry(ptent);
 		if (swp_type(entry) != type)
 			continue;
 
-- 
2.51.0