[PATCH 5/4] mm: Return head pages from find_get_entry

[Matthew Wilcox <willy@infradead.org>]

This was the original destination of the prior patch series.  I have a
few places in the THP patch set which add calls to find_get_entry() and it
annoyed me that I was carefully calling find_subpage() in find_get_entry()
only to immediately call thp_head() to get back to the original subpage.
I'm not sure it's worth applying this as part of the patch series,
which is why I left it out earlier.

There are going to be some other functions which return only head pages.
I currently have a find_get_heads_range_tag() in my tree, which is
probably going to become find_get_entries_range_tag().

--- 8< ---

mm: Return head pages from find_get_entry

All the callers of find_get_entry() call compound_head() to get back to
the head page.  They still do because compound_head() calls are hidden in
such functions as put_page() and lock_page(), but it lets us get rid of
a few calls.

diff --git a/mm/filemap.c b/mm/filemap.c
index f0ae9a6308cb..7e0a7d02e7aa 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1487,9 +1487,9 @@ EXPORT_SYMBOL(page_cache_prev_miss);
 /**
  * find_get_entry - find and get a page cache entry
  * @mapping: the address_space to search
- * @offset: the page cache index
+ * @index: the page cache index
  *
- * Looks up the page cache slot at @mapping & @offset.  If there is a
+ * Looks up the page cache slot at @mapping & @index.  If there is a
  * page cache page, it is returned with an increased refcount.
  *
  * If the slot holds a shadow entry of a previously evicted page, or a
@@ -1497,9 +1497,9 @@ EXPORT_SYMBOL(page_cache_prev_miss);
  *
  * Return: the found page or shadow entry, %NULL if nothing is found.
  */
-struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
+struct page *find_get_entry(struct address_space *mapping, pgoff_t index)
 {
-	XA_STATE(xas, &mapping->i_pages, offset);
+	XA_STATE(xas, &mapping->i_pages, index);
 	struct page *page;
 
 	rcu_read_lock();
@@ -1527,7 +1527,6 @@ struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
 		put_page(page);
 		goto repeat;
 	}
-	page = find_subpage(page, offset);
 out:
 	rcu_read_unlock();
 
@@ -1537,9 +1536,9 @@ struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
 /**
  * find_lock_entry - locate, pin and lock a page cache entry
  * @mapping: the address_space to search
- * @offset: the page cache index
+ * @index: the page cache index
  *
- * Looks up the page cache slot at @mapping & @offset.  If there is a
+ * Looks up the page cache slot at @mapping & @index.  If there is a
  * page cache page, it is returned locked and with an increased
  * refcount.
  *
@@ -1550,21 +1549,22 @@ struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
  *
  * Return: the found page or shadow entry, %NULL if nothing is found.
  */
-struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)
+struct page *find_lock_entry(struct address_space *mapping, pgoff_t index)
 {
 	struct page *page;
 
 repeat:
-	page = find_get_entry(mapping, offset);
+	page = find_get_entry(mapping, index);
 	if (page && !xa_is_value(page)) {
 		lock_page(page);
 		/* Has the page been truncated? */
-		if (unlikely(page_mapping(page) != mapping)) {
+		if (unlikely(page->mapping != mapping)) {
 			unlock_page(page);
 			put_page(page);
 			goto repeat;
 		}
-		VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
+		page = find_subpage(page, index);
+		VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page);
 	}
 	return page;
 }
@@ -1620,12 +1620,13 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 		}
 
 		/* Has the page been truncated? */
-		if (unlikely(compound_head(page)->mapping != mapping)) {
+		if (unlikely(page->mapping != mapping)) {
 			unlock_page(page);
 			put_page(page);
 			goto repeat;
 		}
-		VM_BUG_ON_PAGE(page->index != index, page);
+		VM_BUG_ON_PAGE(page->index !=
+				(index & ~(thp_nr_pages(page) - 1)), page);
 	}
 
 	if (fgp_flags & FGP_ACCESSED)
@@ -1666,7 +1667,7 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 			unlock_page(page);
 	}
 
-	return page;
+	return find_subpage(page, index);
 }
 EXPORT_SYMBOL(pagecache_get_page);
 
diff --git a/mm/mincore.c b/mm/mincore.c
index abc24ca6f0f7..cda857110d44 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -58,8 +58,10 @@ struct page *find_get_swap_page(struct address_space *mapping, pgoff_t index)
 	struct swap_info_struct *si;
 	struct page *page = find_get_entry(mapping, index);
 
-	if (!xa_is_value(page))
+	if (!page)
 		return page;
+	if (!xa_is_value(page))
+		return find_subpage(page, index);
 	if (!IS_ENABLED(CONFIG_SWAP) || !shmem_mapping(mapping))
 		return NULL;