mm/swap: stop using page->private on tail pages for THP_SWAP

Patch series "mm/swap: stop using page->private on tail pages for THP_SWAP
+ cleanups".

This series stops using page->private on tail pages for THP_SWAP, replaces
folio->private by folio->swap for swapcache folios, and starts using
"new_folio" for tail pages that we are splitting to remove the usage of
page->private for swapcache handling completely.


This patch (of 4):

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().

[david@redhat.com: stop using page->private on tail pages for THP_SWAP]
  Link: https://lkml.kernel.org/r/6f0a82a3-6948-20d9-580b-be1dbf415701@redhat.com
Link: https://lkml.kernel.org/r/20230821160849.531668-1-david@redhat.com
Link: https://lkml.kernel.org/r/20230821160849.531668-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>	[arm64]
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 10 insertions, 11 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2b9d8be28361..55cd4bc57b8d 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -322,11 +322,8 @@ 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;
 	};
@@ -347,9 +344,6 @@ 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;
 	};
@@ -374,9 +368,6 @@ 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) ==			\
@@ -385,7 +376,6 @@ 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..e5cf58a1cf9e 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 += folio_page_idx(folio, page);
+	return entry;
+}
+
 static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
 {
 	folio->private = (void *)entry.val;