[RFC v2 05/14] readahead: align index to mapping_min_order in ondemand_ra and force_ra

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

From: Luis Chamberlain <mcgrof@kernel.org>

Align the ra->start and ra->size to mapping_min_order in
ondemand_readahead(), and align the index to mapping_min_order in
force_page_cache_ra(). This will ensure that the folios allocated for
readahead that are added to the page cache are aligned to
mapping_min_order.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 mm/readahead.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 4fa7d0e65706..5e1ec7705c78 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -315,6 +315,7 @@ void force_page_cache_ra(struct readahead_control *ractl,
 	struct file_ra_state *ra = ractl->ra;
 	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 	unsigned long max_pages, index;
+	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
 
 	if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
 		return;
@@ -324,6 +325,13 @@ void force_page_cache_ra(struct readahead_control *ractl,
 	 * be up to the optimal hardware IO size
 	 */
 	index = readahead_index(ractl);
+	if (!IS_ALIGNED(index, min_nrpages)) {
+		unsigned long old_index = index;
+
+		index = round_down(index, min_nrpages);
+		nr_to_read += (old_index - index);
+	}
+
 	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
 	nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
 	while (nr_to_read) {
@@ -332,6 +340,7 @@ void force_page_cache_ra(struct readahead_control *ractl,
 		if (this_chunk > nr_to_read)
 			this_chunk = nr_to_read;
 		ractl->_index = index;
+		VM_BUG_ON(!IS_ALIGNED(index, min_nrpages));
 		do_page_cache_ra(ractl, this_chunk, 0);
 
 		index += this_chunk;
@@ -344,11 +353,20 @@ void force_page_cache_ra(struct readahead_control *ractl,
  * for small size, x 4 for medium, and x 2 for large
  * for 128k (32 page) max ra
  * 1-2 page = 16k, 3-4 page 32k, 5-8 page = 64k, > 8 page = 128k initial
+ *
+ * For higher order address space requirements we ensure no initial reads
+ * are ever less than the min number of pages required.
+ *
+ * We *always* cap the max io size allowed by the device.
  */
-static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
+static unsigned long get_init_ra_size(unsigned long size,
+				      unsigned int min_nrpages,
+				      unsigned long max)
 {
 	unsigned long newsize = roundup_pow_of_two(size);
 
+	newsize = max_t(unsigned long, newsize, min_nrpages);
+
 	if (newsize <= max / 32)
 		newsize = newsize * 4;
 	else if (newsize <= max / 4)
@@ -356,6 +374,8 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 	else
 		newsize = max;
 
+	VM_BUG_ON(newsize & (min_nrpages - 1));
+
 	return newsize;
 }
 
@@ -364,14 +384,16 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
  *  return it as the new window size.
  */
 static unsigned long get_next_ra_size(struct file_ra_state *ra,
+				      unsigned int min_nrpages,
 				      unsigned long max)
 {
-	unsigned long cur = ra->size;
+	unsigned long cur = max(ra->size, min_nrpages);
 
 	if (cur < max / 16)
 		return 4 * cur;
 	if (cur <= max / 2)
 		return 2 * cur;
+
 	return max;
 }
 
@@ -561,7 +583,11 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	unsigned long add_pages;
 	pgoff_t index = readahead_index(ractl);
 	pgoff_t expected, prev_index;
-	unsigned int order = folio ? folio_order(folio) : 0;
+	unsigned int min_order = mapping_min_folio_order(ractl->mapping);
+	unsigned int min_nrpages = mapping_min_folio_nrpages(ractl->mapping);
+	unsigned int order = folio ? folio_order(folio) : min_order;
+
+	VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
 
 	/*
 	 * If the request exceeds the readahead window, allow the read to
@@ -583,8 +609,8 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	expected = round_down(ra->start + ra->size - ra->async_size,
 			1UL << order);
 	if (index == expected || index == (ra->start + ra->size)) {
-		ra->start += ra->size;
-		ra->size = get_next_ra_size(ra, max_pages);
+		ra->start += round_down(ra->size, min_nrpages);
+		ra->size = get_next_ra_size(ra, min_nrpages, max_pages);
 		ra->async_size = ra->size;
 		goto readit;
 	}
@@ -603,13 +629,18 @@ static void ondemand_readahead(struct readahead_control *ractl,
 				max_pages);
 		rcu_read_unlock();
 
+		start = round_down(start, min_nrpages);
+
+		VM_BUG_ON(folio->index & (folio_nr_pages(folio) - 1));
+
 		if (!start || start - index > max_pages)
 			return;
 
 		ra->start = start;
 		ra->size = start - index;	/* old async_size */
+
 		ra->size += req_size;
-		ra->size = get_next_ra_size(ra, max_pages);
+		ra->size = get_next_ra_size(ra, min_nrpages, max_pages);
 		ra->async_size = ra->size;
 		goto readit;
 	}
@@ -646,7 +677,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
 
 initial_readahead:
 	ra->start = index;
-	ra->size = get_init_ra_size(req_size, max_pages);
+	ra->size = get_init_ra_size(req_size, min_nrpages, max_pages);
 	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 
 readit:
@@ -657,7 +688,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	 * Take care of maximum IO pages as above.
 	 */
 	if (index == ra->start && ra->size == ra->async_size) {
-		add_pages = get_next_ra_size(ra, max_pages);
+		add_pages = get_next_ra_size(ra, min_nrpages, max_pages);
 		if (ra->size + add_pages <= max_pages) {
 			ra->async_size = add_pages;
 			ra->size += add_pages;
@@ -668,6 +699,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	}
 
 	ractl->_index = ra->start;
+	VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
 	page_cache_ra_order(ractl, ra, order);
 }
 
-- 
2.43.0