[PATCH v4 04/11] readahead: allocate folios with mapping_min_order in readahead

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

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

page_cache_ra_unbounded() was allocating single pages (0 order folios)
if there was no folio found in an index. Allocate mapping_min_order folios
as we need to guarantee the minimum order if it is set.
When read_pages() is triggered and if a page is already present, check
for truncation and move the ractl->_index by mapping_min_nrpages if that
folio was truncated. This is done to ensure we keep the alignment
requirement while adding a folio to the page cache.

page_cache_ra_order() tries to allocate folio to the page cache with a
higher order if the index aligns with that order. Modify it so that the
order does not go below the mapping_min_order requirement of the page
cache. This function will do the right thing even if the new_order passed
is less than the mapping_min_order.
When adding new folios to the page cache we must also ensure the index
used is aligned to the mapping_min_order as the page cache requires the
index to be aligned to the order of the folio.

readahead_expand() is called from readahead aops to extend the range of
the readahead so this function can assume ractl->_index to be aligned with
min_order.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 mm/readahead.c | 85 +++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 71 insertions(+), 14 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 2361634a84fd..9c193185a836 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -206,9 +206,10 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 		unsigned long nr_to_read, unsigned long lookahead_size)
 {
 	struct address_space *mapping = ractl->mapping;
-	unsigned long index = readahead_index(ractl);
+	unsigned long index = readahead_index(ractl), ra_folio_index;
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
-	unsigned long i = 0;
+	unsigned long i = 0, mark;
+	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
 
 	/*
 	 * Partway through the readahead operation, we will have added
@@ -223,6 +224,22 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 	unsigned int nofs = memalloc_nofs_save();
 
 	filemap_invalidate_lock_shared(mapping);
+	index = mapping_align_start_index(mapping, index);
+
+	/*
+	 * As iterator `i` is aligned to min_nrpages, round_up the
+	 * difference between nr_to_read and lookahead_size to mark the
+	 * index that only has lookahead or "async_region" to set the
+	 * readahead flag.
+	 */
+	ra_folio_index = round_up(readahead_index(ractl) + nr_to_read - lookahead_size,
+				  min_nrpages);
+	mark = ra_folio_index - index;
+	if (index != readahead_index(ractl)) {
+		nr_to_read += readahead_index(ractl) - index;
+		ractl->_index = index;
+	}
+
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
@@ -230,6 +247,8 @@ 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
@@ -239,23 +258,35 @@ 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;
 		}
-		if (i == nr_to_read - lookahead_size)
+		if (i == mark)
 			folio_set_readahead(folio);
 		ractl->_workingset |= folio_test_workingset(folio);
 		ractl->_nr_pages += folio_nr_pages(folio);
@@ -489,12 +520,18 @@ 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;
 	gfp_t gfp = readahead_gfp_mask(mapping);
+	unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));
 
-	if (!mapping_large_folio_support(mapping) || ra->size < 4)
+	/*
+	 * Fallback when size < min_nrpages as each folio should be
+	 * at least min_nrpages anyway.
+	 */
+	if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size)
 		goto fallback;
 
 	limit = min(limit, index + ra->size - 1);
@@ -503,9 +540,18 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		new_order += 2;
 		new_order = min_t(unsigned int, MAX_PAGECACHE_ORDER, new_order);
 		new_order = min_t(unsigned int, new_order, ilog2(ra->size));
+		new_order = max(new_order, min_order);
 	}
 
 	filemap_invalidate_lock_shared(mapping);
+	/*
+	 * If the new_order is greater than min_order and index is
+	 * already aligned to new_order, then this will be noop as index
+	 * aligned to new_order should also be aligned to min_order.
+	 */
+	ractl->_index = mapping_align_start_index(mapping, index);
+	index = readahead_index(ractl);
+
 	while (index <= limit) {
 		unsigned int order = new_order;
 
@@ -513,7 +559,7 @@ 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--;
 		err = ra_alloc_folio(ractl, index, mark, order, gfp);
 		if (err)
@@ -776,8 +822,15 @@ void readahead_expand(struct readahead_control *ractl,
 	struct file_ra_state *ra = ractl->ra;
 	pgoff_t new_index, new_nr_pages;
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+	unsigned long min_nrpages = mapping_min_folio_nrpages(mapping);
+	unsigned int min_order = mapping_min_folio_order(mapping);
 
 	new_index = new_start / PAGE_SIZE;
+	/*
+	 * Readahead code should have aligned the ractl->_index to
+	 * min_nrpages before calling readahead aops.
+	 */
+	VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
 
 	/* Expand the leading edge downwards */
 	while (ractl->_index > new_index) {
@@ -787,9 +840,11 @@ void readahead_expand(struct readahead_control *ractl,
 		if (folio && !xa_is_value(folio))
 			return; /* Folio apparently present */
 
-		folio = filemap_alloc_folio(gfp_mask, 0);
+		folio = filemap_alloc_folio(gfp_mask, min_order);
 		if (!folio)
 			return;
+
+		index = mapping_align_start_index(mapping, index);
 		if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
 			folio_put(folio);
 			return;
@@ -799,7 +854,7 @@ void readahead_expand(struct readahead_control *ractl,
 			ractl->_workingset = true;
 			psi_memstall_enter(&ractl->_pflags);
 		}
-		ractl->_nr_pages++;
+		ractl->_nr_pages += min_nrpages;
 		ractl->_index = folio->index;
 	}
 
@@ -814,9 +869,11 @@ void readahead_expand(struct readahead_control *ractl,
 		if (folio && !xa_is_value(folio))
 			return; /* Folio apparently present */
 
-		folio = filemap_alloc_folio(gfp_mask, 0);
+		folio = filemap_alloc_folio(gfp_mask, min_order);
 		if (!folio)
 			return;
+
+		index = mapping_align_start_index(mapping, index);
 		if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
 			folio_put(folio);
 			return;
@@ -826,10 +883,10 @@ void readahead_expand(struct readahead_control *ractl,
 			ractl->_workingset = true;
 			psi_memstall_enter(&ractl->_pflags);
 		}
-		ractl->_nr_pages++;
+		ractl->_nr_pages += min_nrpages;
 		if (ra) {
-			ra->size++;
-			ra->async_size++;
+			ra->size += min_nrpages;
+			ra->async_size += min_nrpages;
 		}
 	}
 }
-- 
2.34.1