This patch introduces a new address operation, a_ops->ra_folio_order(), which
proposes a new folio order based on the adjusted order for page_cache_sync_ra.

Hence, each filesystem can set the desired minimum order of folio allocation
when requesting fadvise(POSIX_FADV_WILLNEED).

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
 include/linux/fs.h      |  4 ++++
 include/linux/pagemap.h | 12 ++++++++++++
 mm/readahead.c          |  6 ++++--
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c895146c1444..ddab68b7e03b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -472,6 +472,10 @@ struct address_space_operations {
 	void (*is_dirty_writeback) (struct folio *, bool *dirty, bool *wb);
 	int (*error_remove_folio)(struct address_space *, struct folio *);
 
+	/* Min folio order to allocate pages. */
+	unsigned int (*ra_folio_order)(struct address_space *mapping,
+			unsigned int order);
+
 	/* swapfile support */
 	int (*swap_activate)(struct swap_info_struct *sis, struct file *file,
 				sector_t *span);
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 09b581c1d878..e1fe07477220 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -476,6 +476,18 @@ mapping_min_folio_order(const struct address_space *mapping)
 	return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
 }
 
+static inline unsigned int
+mapping_ra_folio_order(struct address_space *mapping, unsigned int order)
+{
+	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+		return 0;
+
+	if (!mapping->a_ops->ra_folio_order)
+		return order;
+
+	return mapping->a_ops->ra_folio_order(mapping, order);
+}
+
 static inline unsigned long
 mapping_min_folio_nrpages(const struct address_space *mapping)
 {
diff --git a/mm/readahead.c b/mm/readahead.c
index 5beaf7803554..8c7d08af6e00 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -592,8 +592,10 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 	 * A start of file, oversized read, or sequential cache miss:
 	 * trivial case: (index - prev_index) == 1
 	 * unaligned reads: (index - prev_index) == 0
+	 * if filesystem sets high-order allocation
 	 */
-	if (!index || req_count > max_pages || index - prev_index <= 1UL) {
+	if (!index || req_count > max_pages || index - prev_index <= 1UL ||
+	    mapping_ra_folio_order(ractl->mapping, 0)) {
 		ra->start = index;
 		ra->size = get_init_ra_size(req_count, max_pages);
 		ra->async_size = ra->size > req_count ? ra->size - req_count :
@@ -627,7 +629,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 	ra->size = min(contig_count + req_count, max_pages);
 	ra->async_size = 1;
 readit:
-	ra->order = 0;
+	ra->order = mapping_ra_folio_order(ractl->mapping, 0);
 	ractl->_index = ra->start;
 	page_cache_ra_order(ractl, ra);
 }
-- 
2.52.0.107.ga0afd4fd5b-goog