[PATCH 1/4] filemap: return pos of first dirty folio from range_has_writeback

[Brian Foster <bfoster@xxxxxxxxxx>]

The iomap layer has a use case that wants to locate the first dirty
or writeback folio in a particular range.
filemap_range_has_writeback() already implements this with the
exception that it only returns a boolean.

Since the _needs_writeback() wrapper is currently the only caller,
tweak has_writeback to take a pointer for the starting offset and
update it to the offset of the first dirty folio found.

Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
---
 include/linux/pagemap.h | 4 ++--
 mm/filemap.c            | 8 +++++---
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index a0a026d2d244..a15131a3fa12 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1217,7 +1217,7 @@ int __filemap_add_folio(struct address_space *mapping, struct folio *folio,
 		pgoff_t index, gfp_t gfp, void **shadowp);
 
 bool filemap_range_has_writeback(struct address_space *mapping,
-				 loff_t start_byte, loff_t end_byte);
+				 loff_t *start_byte, loff_t end_byte);
 
 /**
  * filemap_range_needs_writeback - check if range potentially needs writeback
@@ -1242,7 +1242,7 @@ static inline bool filemap_range_needs_writeback(struct address_space *mapping,
 	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
 	    !mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
 		return false;
-	return filemap_range_has_writeback(mapping, start_byte, end_byte);
+	return filemap_range_has_writeback(mapping, &start_byte, end_byte);
 }
 
 /**
diff --git a/mm/filemap.c b/mm/filemap.c
index 657bcd887fdb..be0a219e8d9e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -636,13 +636,13 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 }
 
 bool filemap_range_has_writeback(struct address_space *mapping,
-				 loff_t start_byte, loff_t end_byte)
+				 loff_t *start_byte, loff_t end_byte)
 {
-	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
+	XA_STATE(xas, &mapping->i_pages, *start_byte >> PAGE_SHIFT);
 	pgoff_t max = end_byte >> PAGE_SHIFT;
 	struct folio *folio;
 
-	if (end_byte < start_byte)
+	if (end_byte < *start_byte)
 		return false;
 
 	rcu_read_lock();
@@ -655,6 +655,8 @@ bool filemap_range_has_writeback(struct address_space *mapping,
 				folio_test_writeback(folio))
 			break;
 	}
+	if (folio)
+		*start_byte = folio_pos(folio);
 	rcu_read_unlock();
 	return folio != NULL;
 }
-- 
2.45.0