[PATCH 09/11] mm/filemap: drop streaming/uncached pages when writeback completes

[Jens Axboe <axboe@kernel.dk>]

If the folio is marked as streaming, drop pages when writeback completes.
Intended to be used with RWF_DONTCACHE, to avoid needing sync writes for
uncached IO.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 mm/filemap.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 77290ac205dc..ec087bab1c97 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1599,6 +1599,27 @@ int folio_wait_private_2_killable(struct folio *folio)
 }
 EXPORT_SYMBOL(folio_wait_private_2_killable);
 
+/*
+ * 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 -
+ * just reset dropbehind for that case and latter completions should invalidate.
+ */
+static void folio_end_dropbehind(struct folio *folio)
+{
+	/*
+	 * Hitting !in_task() should not happen off RWF_DONTCACHE writeback,
+	 * but can happen if normal writeback just happens to find dirty folios
+	 * that were created as part of uncached writeback, and that writeback
+	 * would otherwise not need non-IRQ handling. Just skip the
+	 * invalidation in that case.
+	 */
+	if (in_task() && folio_trylock(folio)) {
+		if (folio->mapping)
+			folio_unmap_invalidate(folio->mapping, folio, 0);
+		folio_unlock(folio);
+	}
+}
+
 /**
  * folio_end_writeback - End writeback against a folio.
  * @folio: The folio.
@@ -1609,6 +1630,8 @@ EXPORT_SYMBOL(folio_wait_private_2_killable);
  */
 void folio_end_writeback(struct folio *folio)
 {
+	bool folio_dropbehind = false;
+
 	VM_BUG_ON_FOLIO(!folio_test_writeback(folio), folio);
 
 	/*
@@ -1630,9 +1653,14 @@ void folio_end_writeback(struct folio *folio)
 	 * reused before the folio_wake_bit().
 	 */
 	folio_get(folio);
+	if (!folio_test_dirty(folio))
+		folio_dropbehind = folio_test_clear_dropbehind(folio);
 	if (__folio_end_writeback(folio))
 		folio_wake_bit(folio, PG_writeback);
 	acct_reclaim_writeback(folio);
+
+	if (folio_dropbehind)
+		folio_end_dropbehind(folio);
 	folio_put(folio);
 }
 EXPORT_SYMBOL(folio_end_writeback);
-- 
2.45.2