Let's stop using page->private on tail pages, making it possible to
just unconditionally reuse that field in the tail pages of large folios.

The remaining usage of the private field for THP_SWAP is in the THP
splitting code (mm/huge_memory.c), that we'll handle separately later.

Update the THP_SWAP documentation and sanity checks in mm_types.h and
__split_huge_page_tail().

Signed-off-by: David Hildenbrand <redacted>
---
 arch/arm64/mm/mteswap.c  |  5 +++--
 include/linux/mm_types.h | 12 +-----------
 include/linux/swap.h     |  9 +++++++++
 mm/huge_memory.c         | 15 ++++++---------
 mm/memory.c              |  2 +-
 mm/rmap.c                |  2 +-
 mm/swap_state.c          |  5 +++--
 mm/swapfile.c            |  4 ++--
 8 files changed, 26 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index cd508ba80ab1..a31833e3ddc5 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c

@@ -33,8 +33,9 @@ int mte_save_tags(struct page *page)

        mte_save_page_tags(page_address(page), tag_storage);

-       /* page_private contains the swap entry.val set in do_swap_page */
-       ret = xa_store(&mte_pages, page_private(page), tag_storage, GFP_KERNEL);
+       /* lookup the swap entry.val from the page */
+       ret = xa_store(&mte_pages, page_swap_entry(page).val, tag_storage,
+                      GFP_KERNEL);
        if (WARN(xa_is_err(ret), "Failed to store MTE tags")) {
                mte_free_tag_storage(tag_storage);
                return xa_err(ret);

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index b9b6c88875b9..61361f1750c3 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -333,11 +333,8 @@ struct folio {
                        atomic_t _pincount;
 #ifdef CONFIG_64BIT
                        unsigned int _folio_nr_pages;
-                       /* 4 byte gap here */
-       /* private: the union with struct page is transitional */
-                       /* Fix THP_SWAP to not use tail->private */
-                       unsigned long _private_1;
 #endif
+       /* private: the union with struct page is transitional */
                };
                struct page __page_1;
        };

@@ -358,9 +355,6 @@ struct folio {
        /* public: */
                        struct list_head _deferred_list;
        /* private: the union with struct page is transitional */
-                       unsigned long _avail_2a;
-                       /* Fix THP_SWAP to not use tail->private */
-                       unsigned long _private_2a;
                };
                struct page __page_2;
        };

@@ -385,9 +379,6 @@ FOLIO_MATCH(memcg_data, memcg_data);
                        offsetof(struct page, pg) + sizeof(struct page))
 FOLIO_MATCH(flags, _flags_1);
 FOLIO_MATCH(compound_head, _head_1);
-#ifdef CONFIG_64BIT
-FOLIO_MATCH(private, _private_1);
-#endif
 #undef FOLIO_MATCH
 #define FOLIO_MATCH(pg, fl)                                            \
        static_assert(offsetof(struct folio, fl) ==                     \

@@ -396,7 +387,6 @@ FOLIO_MATCH(flags, _flags_2);
 FOLIO_MATCH(compound_head, _head_2);
 FOLIO_MATCH(flags, _flags_2a);
 FOLIO_MATCH(compound_head, _head_2a);
-FOLIO_MATCH(private, _private_2a);
 #undef FOLIO_MATCH

 /**

diff --git a/include/linux/swap.h b/include/linux/swap.h
index bb5adc604144..84fe0e94f5cd 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h

@@ -339,6 +339,15 @@ static inline swp_entry_t folio_swap_entry(struct folio *folio)
        return entry;
 }

+static inline swp_entry_t page_swap_entry(struct page *page)
+{
+       struct folio *folio = page_folio(page);
+       swp_entry_t entry = folio_swap_entry(folio);
+
+       entry.val += page - &folio->page;

+       return entry;
+}
+
 static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
 {
        folio->private = (void *)entry.val;

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cc2f65f8cc62..c04702ae71d2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -2446,18 +2446,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
        page_tail->index = head->index + tail;

        /*
-        * page->private should not be set in tail pages with the exception
-        * of swap cache pages that store the swp_entry_t in tail pages.
-        * Fix up and warn once if private is unexpectedly set.
-        *
-        * What of 32-bit systems, on which folio->_pincount overlays
-        * head[1].private?  No problem: THP_SWAP is not enabled on 32-bit, and
-        * pincount must be 0 for folio_ref_freeze() to have succeeded.
+        * page->private should not be set in tail pages. Fix up and warn once
+        * if private is unexpectedly set.
         */
-       if (!folio_test_swapcache(page_folio(head))) {
-               VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
+       if (unlikely(page_tail->private)) {
+               VM_WARN_ON_ONCE_PAGE(true, page_tail);
                page_tail->private = 0;
        }
+       if (PageSwapCache(head))
+               set_page_private(page_tail, (unsigned long)head->private + tail);

        /* Page flags must be visible before we make the page non-compound. */
        smp_wmb();

diff --git a/mm/memory.c b/mm/memory.c
index d003076b218d..ff13242c1589 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -3882,7 +3882,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
                 * changed.
                 */
                if (unlikely(!folio_test_swapcache(folio) ||
-                            page_private(page) != entry.val))
+                            page_swap_entry(page).val != entry.val))
                        goto out_page;

                /*

diff --git a/mm/rmap.c b/mm/rmap.c
index 1f04debdc87a..ec7f8e6c9e48 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -1647,7 +1647,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
                         */
                        dec_mm_counter(mm, mm_counter(&folio->page));
                } else if (folio_test_anon(folio)) {
-                       swp_entry_t entry = { .val = page_private(subpage) };
+                       swp_entry_t entry = page_swap_entry(subpage);
                        pte_t swp_pte;
                        /*
                         * Store the swap location in the pte.

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 01f15139b7d9..2f2417810052 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c

@@ -100,6 +100,7 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,

        folio_ref_add(folio, nr);
        folio_set_swapcache(folio);
+       folio_set_swap_entry(folio, entry);

        do {
                xas_lock_irq(&xas);

@@ -113,7 +114,6 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
                                if (shadowp)
                                        *shadowp = old;
                        }
-                       set_page_private(folio_page(folio, i), entry.val + i);
                        xas_store(&xas, folio);
                        xas_next(&xas);
                }

@@ -154,9 +154,10 @@ void __delete_from_swap_cache(struct folio *folio,
        for (i = 0; i < nr; i++) {
                void *entry = xas_store(&xas, shadow);
                VM_BUG_ON_PAGE(entry != folio, entry);
-               set_page_private(folio_page(folio, i), 0);
                xas_next(&xas);
        }
+       entry.val = 0;
+       folio_set_swap_entry(folio, entry);
        folio_clear_swapcache(folio);
        address_space->nrpages -= nr;
        __node_stat_mod_folio(folio, NR_FILE_PAGES, -nr);

diff --git a/mm/swapfile.c b/mm/swapfile.c
index d46933adf789..bd9d904671b9 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c

@@ -3369,7 +3369,7 @@ struct swap_info_struct *swp_swap_info(swp_entry_t entry)

 struct swap_info_struct *page_swap_info(struct page *page)
 {
-       swp_entry_t entry = { .val = page_private(page) };
+       swp_entry_t entry = page_swap_entry(page);
        return swp_swap_info(entry);
 }

@@ -3384,7 +3384,7 @@ EXPORT_SYMBOL_GPL(swapcache_mapping);

 pgoff_t __page_file_index(struct page *page)
 {
-       swp_entry_t swap = { .val = page_private(page) };
+       swp_entry_t swap = page_swap_entry(page);
        return swp_offset(swap);
 }
 EXPORT_SYMBOL_GPL(__page_file_index);
--

2.41.0