Re: [PATCH v2 1/5] mm/filemap: reduce unnecessary xarray lookups when read cached pages

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 01, 2026 at 01:57:00PM +0800, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@kylinos.cn>
> 
> When reading small amounts of data from the page cache, only a single
> folio is typically returned from filemap_read_get_batch(). In this case,
> calling xas_advance() or xas_next() after adding the folio to the batch
> is unnecessary and only introduces extra branches.

I don't think xas_advance() is all that expensive.  The expensive part
is going to be calling xas_next() and realising that the folio now lies
beyond the required range.  I think you'll get just as much benefit from
doing this:

diff --git a/mm/filemap.c b/mm/filemap.c
index 4e636647100c..578b03bd5514 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2459,11 +2459,14 @@ static void filemap_get_read_batch(struct address_space *mapping,
 	XA_STATE(xas, &mapping->i_pages, index);
 	struct folio *folio;
 
+	if (index > max)
+		return;
+
 	rcu_read_lock();
 	for (folio = xas_load(&xas); folio; folio = xas_next(&xas)) {
 		if (xas_retry(&xas, folio))
 			continue;
-		if (xas.xa_index > max || xa_is_value(folio))
+		if (xa_is_value(folio))
 			break;
 		if (xa_is_sibling(folio))
 			break;
@@ -2480,6 +2483,8 @@ static void filemap_get_read_batch(struct address_space *mapping,
 		if (folio_test_readahead(folio))
 			break;
 		xas_advance(&xas, folio_next_index(folio) - 1);
+		if (xas.index >= max)
+			break;
 		continue;
 put_folio:
 		folio_put(folio);
>