[RFC PATCH 1/3] filemap: Add a helper for filesystems implementing dropbehind

[trondmy@kernel.org]

From: Trond Myklebust <trond.myklebust@hammerspace.com>

Add a helper to allow filesystems to attempt to free the 'dropbehind'
folio.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 include/linux/pagemap.h |  1 +
 mm/filemap.c            | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 26baa78f1ca7..63e2bee9f46b 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1225,6 +1225,7 @@ void folio_wait_writeback(struct folio *folio);
 int folio_wait_writeback_killable(struct folio *folio);
 void end_page_writeback(struct page *page);
 void folio_end_writeback(struct folio *folio);
+void folio_end_dropbehind(struct folio *folio);
 void folio_wait_stable(struct folio *folio);
 void __folio_mark_dirty(struct folio *folio, struct address_space *, int warn);
 void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb);
diff --git a/mm/filemap.c b/mm/filemap.c
index b5e784f34d98..12f694880bb8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1589,6 +1589,22 @@ int folio_wait_private_2_killable(struct folio *folio)
 }
 EXPORT_SYMBOL(folio_wait_private_2_killable);
 
+/*
+ * Helper for filesystems that want to implement dropbehind, and that
+ * need to keep the folio around after folio_end_writeback, e.g. due to
+ * the need to first commit NFS stable writes.
+ */
+void folio_end_dropbehind(struct folio *folio)
+{
+	if (folio_trylock(folio)) {
+		if (folio->mapping && !folio_test_dirty(folio) &&
+		    !folio_test_writeback(folio))
+			folio_unmap_invalidate(folio->mapping, folio, 0);
+		folio_unlock(folio);
+	}
+}
+EXPORT_SYMBOL(folio_end_dropbehind);
+
 /*
  * If folio was marked as dropbehind, then pages should be dropped when writeback
  * completes. Do that now. If we fail, it's likely because of a big folio -
-- 
2.49.0