[PATCH 1/6] filemap: make the folio order calculation shareable

[Daniel Gomez <da.gomez@xxxxxxxxxxx>]

To make the code that clamps the folio order in the __filemap_get_folio
routine reusable to others, move and merge it to the fgf_set_order
new subroutine (mapping_size_order), so when mapping the size at a
given index, the order calculated is already valid and ready to be
used when order is retrieved from fgp_flags with FGF_GET_ORDER.

Signed-off-by: Daniel Gomez <da.gomez@xxxxxxxxxxx>
---
 fs/iomap/buffered-io.c  |  6 ++++--
 include/linux/pagemap.h | 42 ++++++++++++++++++++++++++++++++++++-----
 mm/filemap.c            |  8 --------
 3 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index ae8673ce08b1..bfd9a22a9464 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -546,12 +546,14 @@ EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
 struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
 {
 	fgf_t fgp = FGP_WRITEBEGIN | FGP_NOFS;
+	pgoff_t index = pos >> PAGE_SHIFT;
+	struct address_space *mapping = iter->inode->i_mapping;
 
 	if (iter->flags & IOMAP_NOWAIT)
 		fgp |= FGP_NOWAIT;
-	fgp |= fgf_set_order(len);
+	fgp |= fgf_set_order(mapping, index, len);
 
-	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
+	return __filemap_get_folio(mapping, index,
 			fgp, mapping_gfp_mask(iter->inode->i_mapping));
 }
 EXPORT_SYMBOL_GPL(iomap_get_folio);
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 351c3b7f93a1..7af5636eb32a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -576,6 +576,39 @@ typedef unsigned int __bitwise fgf_t;
 
 #define FGP_WRITEBEGIN		(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
 
+/**
+ * mapping_size_order - Get maximum folio order for the given file size.
+ * @mapping: Target address_space.
+ * @index: The page index.
+ * @size: The suggested size of the folio to create.
+ *
+ * This returns a high order for folios (when supported) based on the file size
+ * which the mapping currently allows at the given index. The index is relevant
+ * due to alignment considerations the mapping might have. The returned order
+ * may be less than the size passed.
+ *
+ * Return: The order.
+ */
+static inline unsigned int mapping_size_order(struct address_space *mapping,
+						 pgoff_t index, size_t size)
+{
+	unsigned int order = ilog2(size);
+
+	if ((order <= PAGE_SHIFT) || (!mapping_large_folio_support(mapping)))
+		return 0;
+	else
+		order = order - PAGE_SHIFT;
+
+	/* If we're not aligned, allocate a smaller folio */
+	if (index & ((1UL << order) - 1))
+		order = __ffs(index);
+
+	order = min_t(size_t, order, MAX_PAGECACHE_ORDER);
+
+	/* Order-1 not supported due to THP dependency */
+	return (order == 1) ? 0 : order;
+}
+
 /**
  * fgf_set_order - Encode a length in the fgf_t flags.
  * @size: The suggested size of the folio to create.
@@ -587,13 +620,12 @@ typedef unsigned int __bitwise fgf_t;
  * due to alignment constraints, memory pressure, or the presence of
  * other folios at nearby indices.
  */
-static inline fgf_t fgf_set_order(size_t size)
+static inline fgf_t fgf_set_order(struct address_space *mapping, pgoff_t index,
+				  size_t size)
 {
-	unsigned int shift = ilog2(size);
+	unsigned int order = mapping_size_order(mapping, index, size);
 
-	if (shift <= PAGE_SHIFT)
-		return 0;
-	return (__force fgf_t)((shift - PAGE_SHIFT) << 26);
+	return (__force fgf_t)(order << 26);
 }
 
 void *filemap_get_entry(struct address_space *mapping, pgoff_t index);
diff --git a/mm/filemap.c b/mm/filemap.c
index 582f5317ff71..e285fffa9bcf 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1917,14 +1917,6 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 		if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP))))
 			fgp_flags |= FGP_LOCK;
 
-		if (!mapping_large_folio_support(mapping))
-			order = 0;
-		if (order > MAX_PAGECACHE_ORDER)
-			order = MAX_PAGECACHE_ORDER;
-		/* If we're not aligned, allocate a smaller folio */
-		if (index & ((1UL << order) - 1))
-			order = __ffs(index);
-
 		do {
 			gfp_t alloc_gfp = gfp;
 
-- 
2.39.2