[PATCH v2 7/8] mm: drop page_index/page_file_offset and convert swap helpers to use folio

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

There are four helpers for retrieving the page index within address
space, or offset within mapped file:

- page_index
- page_file_offset
- folio_index (equivalence of page_index)
- folio_file_pos (equivalence of page_file_offset)

And they are only needed for mixed usage of swap cache & page cache (eg.
migration, huge memory split). Else users can just use folio->index or
folio_pos.

This commit drops page_index and page_file_offset as we have eliminated
all users, and converts folio_index and folio_file_pos (they were simply
wrappers of page_index and page_file_offset, and implemented in a not
very clean way) to use folio internally.

After this commit, there will be only two helpers for users that may
encounter mixed usage of swap cache and page cache:

- folio_index (calls __folio_swap_cache_index for swap cache folio)
- folio_file_pos (calls __folio_swap_dev_pos for swap cache folio)

The index in swap cache and offset in swap device are still basically
the same thing, but will be different in following commits.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/mm.h      | 13 -------------
 include/linux/pagemap.h | 19 +++++++++----------
 mm/swapfile.c           | 13 +++++++++----
 3 files changed, 18 insertions(+), 27 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0436b919f1c7..797480e76c9c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2245,19 +2245,6 @@ static inline void *folio_address(const struct folio *folio)
 	return page_address(&folio->page);
 }
 
-extern pgoff_t __page_file_index(struct page *page);
-
-/*
- * Return the pagecache index of the passed page.  Regular pagecache pages
- * use ->index whereas swapcache pages use swp_offset(->private)
- */
-static inline pgoff_t page_index(struct page *page)
-{
-	if (unlikely(PageSwapCache(page)))
-		return __page_file_index(page);
-	return page->index;
-}
-
 /*
  * Return true only if the page has been allocated with
  * ALLOC_NO_WATERMARKS and the low watermark was not
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2df35e65557d..a7d025571ee6 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -780,7 +780,7 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
 			mapping_gfp_mask(mapping));
 }
 
-#define swapcache_index(folio)	__page_file_index(&(folio)->page)
+extern pgoff_t __folio_swap_cache_index(struct folio *folio);
 
 /**
  * folio_index - File index of a folio.
@@ -795,9 +795,9 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
  */
 static inline pgoff_t folio_index(struct folio *folio)
 {
-        if (unlikely(folio_test_swapcache(folio)))
-                return swapcache_index(folio);
-        return folio->index;
+	if (unlikely(folio_test_swapcache(folio)))
+		return __folio_swap_cache_index(folio);
+	return folio->index;
 }
 
 /**
@@ -920,11 +920,6 @@ static inline loff_t page_offset(struct page *page)
 	return ((loff_t)page->index) << PAGE_SHIFT;
 }
 
-static inline loff_t page_file_offset(struct page *page)
-{
-	return ((loff_t)page_index(page)) << PAGE_SHIFT;
-}
-
 /**
  * folio_pos - Returns the byte position of this folio in its file.
  * @folio: The folio.
@@ -934,6 +929,8 @@ static inline loff_t folio_pos(struct folio *folio)
 	return page_offset(&folio->page);
 }
 
+extern loff_t __folio_swap_dev_pos(struct folio *folio);
+
 /**
  * folio_file_pos - Returns the byte position of this folio in its file.
  * @folio: The folio.
@@ -943,7 +940,9 @@ static inline loff_t folio_pos(struct folio *folio)
  */
 static inline loff_t folio_file_pos(struct folio *folio)
 {
-	return page_file_offset(&folio->page);
+	if (unlikely(folio_test_swapcache(folio)))
+		return __folio_swap_dev_pos(folio);
+	return ((loff_t)folio->index << PAGE_SHIFT);
 }
 
 /*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4919423cce76..2387f5e131d7 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3419,12 +3419,17 @@ struct address_space *swapcache_mapping(struct folio *folio)
 }
 EXPORT_SYMBOL_GPL(swapcache_mapping);
 
-pgoff_t __page_file_index(struct page *page)
+pgoff_t __folio_swap_cache_index(struct folio *folio)
 {
-	swp_entry_t swap = page_swap_entry(page);
-	return swp_offset(swap);
+	return swp_offset(folio->swap);
 }
-EXPORT_SYMBOL_GPL(__page_file_index);
+EXPORT_SYMBOL_GPL(__folio_swap_cache_index);
+
+loff_t __folio_swap_dev_pos(struct folio *folio)
+{
+	return swap_dev_pos(folio->swap);
+}
+EXPORT_SYMBOL_GPL(__folio_swap_dev_pos);
 
 /*
  * add_swap_count_continuation - called when a swap count is duplicated
-- 
2.44.0