[RFC PATCH v4 3/3] mm: Make filemap_release_folio() better inform shrink_folio_list()

[David Howells <dhowells@redhat.com>]

Make filemap_release_folio() return one of three values:

 (0) FILEMAP_CANT_RELEASE_FOLIO

     Couldn't release the folio's private data, so the folio can't itself
     be released.

 (1) FILEMAP_RELEASED_FOLIO

     The private data on the folio was released and the folio can be
     released.

 (2) FILEMAP_FOLIO_HAD_NO_PRIVATE

     There was no private data on the folio and the folio can be released.

The first must be zero so that existing tests of !filemap_release_folio()
continue to work as expected; similarly the other two must both be non-zero
so that existing tests of filemap_release_folio() continue to work as
expected.

Using this, make shrink_folio_list() choose which of three cases to follow
based on the return from filemap_release_folio() rather than testing the
folio's private bit itself.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Dave Wysochanski <dwysocha@redhat.com>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: linux-cachefs@redhat.com
cc: linux-cifs@vger.kernel.org
cc: linux-afs@lists.infradead.org
cc: v9fs-developer@lists.sourceforge.net
cc: ceph-devel@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/1459152.1669208550@warthog.procyon.org.uk/ # v3
---

 include/linux/pagemap.h |    7 ++++++-
 mm/filemap.c            |   20 ++++++++++++++------
 mm/vmscan.c             |   29 +++++++++++++++--------------
 3 files changed, 35 insertions(+), 21 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 9a824b43c6af..b763182b6d3f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1124,7 +1124,12 @@ void replace_page_cache_page(struct page *old, struct page *new);
 void delete_from_page_cache_batch(struct address_space *mapping,
 				  struct folio_batch *fbatch);
 int try_to_release_page(struct page *page, gfp_t gfp);
-bool filemap_release_folio(struct folio *folio, gfp_t gfp);
+enum filemap_released_folio {
+	FILEMAP_CANT_RELEASE_FOLIO	= 0, /* (This must be 0) Release failed */
+	FILEMAP_RELEASED_FOLIO		= 1, /* Folio's private data released */
+	FILEMAP_FOLIO_HAD_NO_PRIVATE	= 2, /* Folio had no private data */
+};
+enum filemap_released_folio filemap_release_folio(struct folio *folio, gfp_t gfp);
 loff_t mapping_seek_hole_data(struct address_space *, loff_t start, loff_t end,
 		int whence);
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 93757247cd11..859831c70439 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3934,20 +3934,28 @@ EXPORT_SYMBOL(generic_file_write_iter);
  * this page (__GFP_IO), and whether the call may block
  * (__GFP_RECLAIM & __GFP_FS).
  *
- * Return: %true if the release was successful, otherwise %false.
+ * Return: %FILEMAP_RELEASED_FOLIO if the release was successful,
+ * %FILEMAP_CANT_RELEASE_FOLIO if the private data couldn't be released and
+ * %FILEMAP_FOLIO_HAD_NO_PRIVATE if there was no private data.
  */
-bool filemap_release_folio(struct folio *folio, gfp_t gfp)
+enum filemap_released_folio filemap_release_folio(struct folio *folio,
+						  gfp_t gfp)
 {
 	struct address_space * const mapping = folio->mapping;
+	bool released;
 
 	BUG_ON(!folio_test_locked(folio));
 	if (!folio_needs_release(folio))
-		return true;
+		return FILEMAP_FOLIO_HAD_NO_PRIVATE;
 	if (folio_test_writeback(folio))
-		return false;
+		return FILEMAP_CANT_RELEASE_FOLIO;
 
 	if (mapping && mapping->a_ops->release_folio)
-		return mapping->a_ops->release_folio(folio, gfp);
-	return try_to_free_buffers(folio);
+		released = mapping->a_ops->release_folio(folio, gfp);
+	else
+		released = try_to_free_buffers(folio);
+
+	return released ?
+		FILEMAP_RELEASED_FOLIO : FILEMAP_CANT_RELEASE_FOLIO;
 }
 EXPORT_SYMBOL(filemap_release_folio);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b9316f447238..d5c7b3be9947 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1978,25 +1978,26 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 * (refcount == 1) it can be freed.  Otherwise, leave
 		 * the folio on the LRU so it is swappable.
 		 */
-		if (folio_needs_release(folio)) {
-			if (!filemap_release_folio(folio, sc->gfp_mask))
-				goto activate_locked;
+		switch (filemap_release_folio(folio, sc->gfp_mask)) {
+		case FILEMAP_CANT_RELEASE_FOLIO:
+			goto activate_locked;
+		case FILEMAP_RELEASED_FOLIO:
 			if (!mapping && folio_ref_count(folio) == 1) {
 				folio_unlock(folio);
 				if (folio_put_testzero(folio))
 					goto free_it;
-				else {
-					/*
-					 * rare race with speculative reference.
-					 * the speculative reference will free
-					 * this folio shortly, so we may
-					 * increment nr_reclaimed here (and
-					 * leave it off the LRU).
-					 */
-					nr_reclaimed += nr_pages;
-					continue;
-				}
+				/*
+				 * rare race with speculative reference.  the
+				 * speculative reference will free this folio
+				 * shortly, so we may increment nr_reclaimed
+				 * here (and leave it off the LRU).
+				 */
+				nr_reclaimed += nr_pages;
+				continue;
 			}
+			break;
+		case FILEMAP_FOLIO_HAD_NO_PRIVATE:
+			break;
 		}
 
 		if (folio_test_anon(folio) && !folio_test_swapbacked(folio)) {