[PATCH v14 138/138] mm/readahead: Add multi-page folio readahead

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

If the filesystem supports multi-page folios, allocate larger pages in
the readahead code when it seems worth doing.  The heuristic for choosing
larger page sizes will surely need some tuning, but this aggressive
ramp-up has been good for testing.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/readahead.c | 102 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 95 insertions(+), 7 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index e1df44ad57ed..27e76cc2a9ba 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -149,7 +149,7 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
 
 	blk_finish_plug(&plug);
 
-	BUG_ON(!list_empty(pages));
+	BUG_ON(pages && !list_empty(pages));
 	BUG_ON(readahead_count(rac));
 
 out:
@@ -430,11 +430,99 @@ static int try_context_readahead(struct address_space *mapping,
 	return 1;
 }
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
+		pgoff_t mark, unsigned int order, gfp_t gfp)
+{
+	int err;
+	struct folio *folio = filemap_alloc_folio(gfp, order);
+
+	if (!folio)
+		return -ENOMEM;
+	if (mark - index < (1UL << order))
+		folio_set_readahead(folio);
+	err = filemap_add_folio(ractl->mapping, folio, index, gfp);
+	if (err)
+		folio_put(folio);
+	else
+		ractl->_nr_pages += 1UL << order;
+	return err;
+}
+
+static void page_cache_ra_order(struct readahead_control *ractl,
+		struct file_ra_state *ra, unsigned int new_order)
+{
+	struct address_space *mapping = ractl->mapping;
+	pgoff_t index = readahead_index(ractl);
+	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
+	pgoff_t mark = index + ra->size - ra->async_size;
+	int err = 0;
+	gfp_t gfp = readahead_gfp_mask(mapping);
+
+	if (!mapping_thp_support(mapping) || ra->size < 4)
+		goto fallback;
+
+	limit = min(limit, index + ra->size - 1);
+
+	/* Grow page size up to PMD size */
+	if (new_order < HPAGE_PMD_ORDER) {
+		new_order += 2;
+		if (new_order > HPAGE_PMD_ORDER)
+			new_order = HPAGE_PMD_ORDER;
+		while ((1 << new_order) > ra->size)
+			new_order--;
+	}
+
+	while (index <= limit) {
+		unsigned int order = new_order;
+
+		/* Align with smaller pages if needed */
+		if (index & ((1UL << order) - 1)) {
+			order = __ffs(index);
+			if (order == 1)
+				order = 0;
+		}
+		/* Don't allocate pages past EOF */
+		while (index + (1UL << order) - 1 > limit) {
+			if (--order == 1)
+				order = 0;
+		}
+		err = ra_alloc_folio(ractl, index, mark, order, gfp);
+		if (err)
+			break;
+		index += 1UL << order;
+	}
+
+	if (index > limit) {
+		ra->size += index - limit - 1;
+		ra->async_size += index - limit - 1;
+	}
+
+	read_pages(ractl, NULL, false);
+
+	/*
+	 * If there were already pages in the page cache, then we may have
+	 * left some gaps.  Let the regular readahead code take care of this
+	 * situation.
+	 */
+	if (!err)
+		return;
+fallback:
+	do_page_cache_ra(ractl, ra->size, ra->async_size);
+}
+#else
+static void page_cache_ra_order(struct readahead_control *ractl,
+		struct file_ra_state *ra, unsigned int order)
+{
+	do_page_cache_ra(ractl, ra->size, ra->async_size);
+}
+#endif
+
 /*
  * A minimal readahead algorithm for trivial sequential/random reads.
  */
 static void ondemand_readahead(struct readahead_control *ractl,
-		bool hit_readahead_marker, unsigned long req_size)
+		struct folio *folio, unsigned long req_size)
 {
 	struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
 	struct file_ra_state *ra = ractl->ra;
@@ -469,12 +557,12 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	}
 
 	/*
-	 * Hit a marked page without valid readahead state.
+	 * Hit a marked folio without valid readahead state.
 	 * E.g. interleaved reads.
 	 * Query the pagecache for async_size, which normally equals to
 	 * readahead size. Ramp it up and use it as the new readahead size.
 	 */
-	if (hit_readahead_marker) {
+	if (folio) {
 		pgoff_t start;
 
 		rcu_read_lock();
@@ -547,7 +635,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	}
 
 	ractl->_index = ra->start;
-	do_page_cache_ra(ractl, ra->size, ra->async_size);
+	page_cache_ra_order(ractl, ra, folio ? folio_order(folio) : 0);
 }
 
 void page_cache_sync_ra(struct readahead_control *ractl,
@@ -575,7 +663,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 	}
 
 	/* do read-ahead */
-	ondemand_readahead(ractl, false, req_count);
+	ondemand_readahead(ractl, NULL, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_ra);
 
@@ -604,7 +692,7 @@ void page_cache_async_ra(struct readahead_control *ractl,
 		return;
 
 	/* do read-ahead */
-	ondemand_readahead(ractl, true, req_count);
+	ondemand_readahead(ractl, folio, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_ra);
 
-- 
2.30.2