[PATCH v3 1/7] filemap: add helper to look up dirty folios in a range

[Brian Foster <bfoster@redhat.com>]

Add a new filemap_get_folios_dirty() helper to look up existing dirty
folios in a range and add them to a folio_batch. This is to support
optimization of certain iomap operations that only care about dirty
folios in a target range. For example, zero range only zeroes the subset
of dirty pages over unwritten mappings, seek hole/data may use similar
logic in the future, etc.

Note that the helper is intended for use under internal fs locks.
Therefore it trylocks folios in order to filter out clean folios.
This loosely follows the logic from filemap_range_has_writeback().

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/pagemap.h |  2 ++
 mm/filemap.c            | 58 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e63fbfbd5b0f..fb83ddf26621 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -941,6 +941,8 @@ unsigned filemap_get_folios_contig(struct address_space *mapping,
 		pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
 unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
 		pgoff_t end, xa_mark_t tag, struct folio_batch *fbatch);
+unsigned filemap_get_folios_dirty(struct address_space *mapping,
+		pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
 
 /*
  * Returns locked page at given index in given cache, creating it if needed.
diff --git a/mm/filemap.c b/mm/filemap.c
index bada249b9fb7..2171b7f689b0 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2334,6 +2334,64 @@ unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
 }
 EXPORT_SYMBOL(filemap_get_folios_tag);
 
+/**
+ * filemap_get_folios_dirty - Get a batch of dirty folios
+ * @mapping:	The address_space to search
+ * @start:	The starting folio index
+ * @end:	The final folio index (inclusive)
+ * @fbatch:	The batch to fill
+ *
+ * filemap_get_folios_dirty() works exactly like filemap_get_folios(), except
+ * the returned folios are presumed to be dirty or undergoing writeback. Dirty
+ * state is presumed because we don't block on folio lock nor want to miss
+ * folios. Callers that need to can recheck state upon locking the folio.
+ *
+ * This may not return all dirty folios if the batch gets filled up.
+ *
+ * Return: The number of folios found.
+ * Also update @start to be positioned for traversal of the next folio.
+ */
+unsigned filemap_get_folios_dirty(struct address_space *mapping, pgoff_t *start,
+			pgoff_t end, struct folio_batch *fbatch)
+{
+	XA_STATE(xas, &mapping->i_pages, *start);
+	struct folio *folio;
+
+	rcu_read_lock();
+	while ((folio = find_get_entry(&xas, end, XA_PRESENT)) != NULL) {
+		if (xa_is_value(folio))
+			continue;
+		if (folio_trylock(folio)) {
+			bool clean = !folio_test_dirty(folio) &&
+				     !folio_test_writeback(folio);
+			folio_unlock(folio);
+			if (clean) {
+				folio_put(folio);
+				continue;
+			}
+		}
+		if (!folio_batch_add(fbatch, folio)) {
+			unsigned long nr = folio_nr_pages(folio);
+			*start = folio->index + nr;
+			goto out;
+		}
+	}
+	/*
+	 * We come here when there is no folio beyond @end. We take care to not
+	 * overflow the index @start as it confuses some of the callers. This
+	 * breaks the iteration when there is a folio at index -1 but that is
+	 * already broke anyway.
+	 */
+	if (end == (pgoff_t)-1)
+		*start = (pgoff_t)-1;
+	else
+		*start = end + 1;
+out:
+	rcu_read_unlock();
+
+	return folio_batch_count(fbatch);
+}
+
 /*
  * CD/DVDs are error prone. When a medium error occurs, the driver may fail
  * a _large_ part of the i/o request. Imagine the worst scenario:
-- 
2.50.0