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

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On 2/13/24 10:37, Pankaj Raghav (Samsung) wrote:
From: Luis Chamberlain <mcgrof@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
---
  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 */
+

Stale whitespace.

  		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);
  }
Otherwise looks good.

Cheers,

Hannes





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