Re: [PATCH 03/13] filemap: align the index to mapping_min_order in the page cache

[Matthew Wilcox]

On Mon, Feb 26, 2024 at 10:49:26AM +0100, Pankaj Raghav (Samsung) wrote:
> From: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> 
> Supporting mapping_min_order implies that we guarantee each folio in the
> page cache has at least an order of mapping_min_order. So when adding new
> folios to the page cache we must ensure the index used is aligned to the
> mapping_min_order as the page cache requires the index to be aligned to
> the order of the folio.

This seems like a remarkably complicated way of achieving:

diff --git a/mm/filemap.c b/mm/filemap.c
index 5603ced05fb7..36105dad4440 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2427,9 +2427,11 @@ static int filemap_update_page(struct kiocb *iocb,
 }
 
 static int filemap_create_folio(struct file *file,
-		struct address_space *mapping, pgoff_t index,
+		struct address_space *mapping, loff_t pos,
 		struct folio_batch *fbatch)
 {
+	pgoff_t index;
+	unsigned int min_order;
 	struct folio *folio;
 	int error;
 
@@ -2451,6 +2453,8 @@ static int filemap_create_folio(struct file *file,
 	 * well to keep locking rules simple.
 	 */
 	filemap_invalidate_lock_shared(mapping);
+	min_order = mapping_min_folio_order(mapping);
+	index = (pos >> (min_order + PAGE_SHIFT)) << min_order;
 	error = filemap_add_folio(mapping, folio, index,
 			mapping_gfp_constraint(mapping, GFP_KERNEL));
 	if (error == -EEXIST)
@@ -2511,8 +2515,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
 			return -EAGAIN;
-		err = filemap_create_folio(filp, mapping,
-				iocb->ki_pos >> PAGE_SHIFT, fbatch);
+		err = filemap_create_folio(filp, mapping, iocb->ki_pos, fbatch);
 		if (err == AOP_TRUNCATED_PAGE)
 			goto retry;
 		return err;