mm: add pfn_swap_entry_folio()

Patch series "mm: convert mm counter to take a folio", v3.

Make sure all mm_counter() and mm_counter_file() callers have a folio,
then convert mm counter functions to take a folio, which saves some
compound_head() calls.


This patch (of 10):  

Thanks to the compound_head() hidden inside PageLocked(), this saves a
call to compound_head() over calling page_folio(pfn_swap_entry_to_page())

Link: https://lkml.kernel.org/r/20240111152429.3374566-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20240111152429.3374566-2-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swapops.h

@@ -468,6 +468,19 @@ static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
 	return p;
 }
 
+static inline struct folio *pfn_swap_entry_folio(swp_entry_t entry)
+{
+	struct folio *folio = pfn_folio(swp_offset_pfn(entry));
+
+	/*
+	 * Any use of migration entries may only occur while the
+	 * corresponding folio is locked
+	 */
+	BUG_ON(is_migration_entry(entry) && !folio_test_locked(folio));
+
+	return folio;
+}
+
 /*
  * A pfn swap entry is a special type of swap entry that always has a pfn stored
  * in the swap offset. They are used to represent unaddressable device memory

mm/filemap.c

@@ -1354,7 +1354,7 @@ void migration_entry_wait_on_locked(swp_entry_t entry, spinlock_t *ptl)
 	unsigned long pflags;
 	bool in_thrashing;
 	wait_queue_head_t *q;
-	struct folio *folio = page_folio(pfn_swap_entry_to_page(entry));
+	struct folio *folio = pfn_swap_entry_folio(entry);
 
 	q = folio_waitqueue(folio);
 	if (!folio_test_uptodate(folio) && folio_test_workingset(folio)) {

mm/huge_memory.c

@@ -2045,7 +2045,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 	if (is_swap_pmd(*pmd)) {
 		swp_entry_t entry = pmd_to_swp_entry(*pmd);
-		struct folio *folio = page_folio(pfn_swap_entry_to_page(entry));
+		struct folio *folio = pfn_swap_entry_folio(entry);
 		pmd_t newpmd;
 
 		VM_BUG_ON(!is_pmd_migration_entry(*pmd));