[RFC v2 07/14] readahead: allocate folios with mapping_min_order in ra_(unbounded|order)

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

From: Pankaj Raghav <p.raghav@samsung.com>

Allocate folios with at least mapping_min_order in
page_cache_ra_unbounded() and page_cache_ra_order() as we need to
guarantee a minimum order in the page cache.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 mm/readahead.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 13b62cbd3b79..a361fba18674 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -214,6 +214,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 	unsigned long index = readahead_index(ractl);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
 	unsigned long i = 0;
+	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
 
 	/*
 	 * Partway through the readahead operation, we will have added
@@ -235,6 +236,8 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 		struct folio *folio = xa_load(&mapping->i_pages, index + i);
 
 		if (folio && !xa_is_value(folio)) {
+			long nr_pages = folio_nr_pages(folio);
+
 			/*
 			 * Page already present?  Kick off the current batch
 			 * of contiguous pages before continuing with the
@@ -244,19 +247,31 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 			 * not worth getting one just for that.
 			 */
 			read_pages(ractl);
-			ractl->_index += folio_nr_pages(folio);
+
+			/*
+			 * Move the ractl->_index by at least min_pages
+			 * if the folio got truncated to respect the
+			 * alignment constraint in the page cache.
+			 *
+			 */
+			if (mapping != folio->mapping)
+				nr_pages = min_nrpages;
+
+			VM_BUG_ON_FOLIO(nr_pages < min_nrpages, folio);
+			ractl->_index += nr_pages;
 			i = ractl->_index + ractl->_nr_pages - index;
 			continue;
 		}
 
-		folio = filemap_alloc_folio(gfp_mask, 0);
+		folio = filemap_alloc_folio(gfp_mask,
+					    mapping_min_folio_order(mapping));
 		if (!folio)
 			break;
 		if (filemap_add_folio(mapping, folio, index + i,
 					gfp_mask) < 0) {
 			folio_put(folio);
 			read_pages(ractl);
-			ractl->_index++;
+			ractl->_index += min_nrpages;
 			i = ractl->_index + ractl->_nr_pages - index;
 			continue;
 		}
@@ -516,6 +531,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 {
 	struct address_space *mapping = ractl->mapping;
 	pgoff_t index = readahead_index(ractl);
+	unsigned int min_order = mapping_min_folio_order(mapping);
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
 	int err = 0;
@@ -542,11 +558,17 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		if (index & ((1UL << order) - 1))
 			order = __ffs(index);
 		/* Don't allocate pages past EOF */
-		while (index + (1UL << order) - 1 > limit)
+		while (order > min_order && index + (1UL << order) - 1 > limit)
 			order--;
 		/* THP machinery does not support order-1 */
 		if (order == 1)
 			order = 0;
+
+		if (order < min_order)
+			order = min_order;
+
+		VM_BUG_ON(index & ((1UL << order) - 1));
+
 		err = ra_alloc_folio(ractl, index, mark, order, gfp);
 		if (err)
 			break;
-- 
2.43.0