mm: teach release_pages() to take an array of encoded page pointers too

release_pages() already could take either an array of page pointers, or an
array of folio pointers.  Expand it to also accept an array of encoded
page pointers, which is what both the existing mlock() use and the
upcoming mmu_gather use of encoded page pointers wants.

Note that release_pages() won't actually use, or react to, any extra
encoded bits.  Instead, this is very much a case of "I have walked the
array of encoded pages and done everything the extra bits tell me to do,
now release it all".

Also, while the "either page or folio pointers" dual use was handled with
a cast of the pointer in "release_folios()", this takes a slightly
different approach and uses the "transparent union" attribute to describe
the set of arguments to the function:

  https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html

which has been supported by gcc forever, but the kernel hasn't used
before.

That allows us to avoid using various wrappers with casts, and just use
the same function regardless of use.

Link: https://lkml.kernel.org/r/20221109203051.1835763-2-torvalds@linux-foundation.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/mm/swap.c b/mm/swap.c
index b9a6817e07ff..70e2063ef43a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -981,22 +981,30 @@ void lru_cache_disable(void)
 
 /**
  * release_pages - batched put_page()
- * @pages: array of pages to release
+ * @arg: array of pages to release
  * @nr: number of pages
  *
- * Decrement the reference count on all the pages in @pages.  If it
+ * Decrement the reference count on all the pages in @arg.  If it
  * fell to zero, remove the page from the LRU and free it.
+ *
+ * Note that the argument can be an array of pages, encoded pages,
+ * or folio pointers. We ignore any encoded bits, and turn any of
+ * them into just a folio that gets free'd.
  */
-void release_pages(struct page **pages, int nr)
+void release_pages(release_pages_arg arg, int nr)
 {
 	int i;
+	struct encoded_page **encoded = arg.encoded_pages;
 	LIST_HEAD(pages_to_free);
 	struct lruvec *lruvec = NULL;
 	unsigned long flags = 0;
 	unsigned int lock_batch;
 
 	for (i = 0; i < nr; i++) {
-		struct folio *folio = page_folio(pages[i]);
+		struct folio *folio;
+
+		/* Turn any of the argument types into a folio */
+		folio = page_folio(encoded_page_ptr(encoded[i]));
 
 		/*
 		 * Make sure the IRQ-safe lock-holding time does not get