[RFC 1/4] fs: Allow fine-grained control of folio sizes

[Pankaj Raghav <p.raghav@xxxxxxxxxxx>]

From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>

Some filesystems want to be able to limit the maximum size of folios,
and some want to be able to ensure that folios are at least a certain
size.  Add mapping_set_folio_orders() to allow this level of control
(although it is not yet honoured).

[Pankaj]: added mapping_min_folio_order()

Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/pagemap.h | 46 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 716953ee1ebd..462c36c9dd88 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -202,9 +202,15 @@ enum mapping_flags {
 	AS_EXITING	= 4, 	/* final truncate in progress */
 	/* writeback related tags are not used */
 	AS_NO_WRITEBACK_TAGS = 5,
-	AS_LARGE_FOLIO_SUPPORT = 6,
+	AS_FOLIO_ORDER_MIN = 8,
+	AS_FOLIO_ORDER_MAX = 13,
+	/* 8-17 are used for FOLIO_ORDER */
 };
 
+#define AS_FOLIO_ORDER_MIN_MASK	0x00001f00
+#define AS_FOLIO_ORDER_MAX_MASK 0x0002e000
+#define AS_FOLIO_ORDER_MASK (AS_FOLIO_ORDER_MIN_MASK | AS_FOLIO_ORDER_MAX_MASK)
+
 /**
  * mapping_set_error - record a writeback error in the address_space
  * @mapping: the mapping in which an error should be set
@@ -294,6 +300,29 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
 	m->gfp_mask = mask;
 }
 
+/**
+ * mapping_set_folio_orders() - Set the range of folio sizes supported.
+ * @mapping: The file.
+ * @min: Minimum folio order (between 0-31 inclusive).
+ * @max: Maximum folio order (between 0-31 inclusive).
+ *
+ * The filesystem should call this function in its inode constructor to
+ * indicate which sizes of folio the VFS can use to cache the contents
+ * of the file.  This should only be used if the filesystem needs special
+ * handling of folio sizes (ie there is something the core cannot know).
+ * Do not tune it based on, eg, i_size.
+ *
+ * Context: This should not be called while the inode is active as it
+ * is non-atomic.
+ */
+static inline void mapping_set_folio_orders(struct address_space *mapping,
+		unsigned int min, unsigned int max)
+{
+	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
+			(min << AS_FOLIO_ORDER_MIN) |
+			(max << AS_FOLIO_ORDER_MAX);
+}
+
 /**
  * mapping_set_large_folios() - Indicate the file supports large folios.
  * @mapping: The file.
@@ -307,7 +336,17 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
  */
 static inline void mapping_set_large_folios(struct address_space *mapping)
 {
-	__set_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
+	mapping_set_folio_orders(mapping, 0, 31);
+}
+
+static inline unsigned mapping_max_folio_order(struct address_space *mapping)
+{
+	return (mapping->flags & AS_FOLIO_ORDER_MAX_MASK) >> AS_FOLIO_ORDER_MAX;
+}
+
+static inline unsigned mapping_min_folio_order(struct address_space *mapping)
+{
+	return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
 }
 
 /*
@@ -316,8 +355,7 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
  */
 static inline bool mapping_large_folio_support(struct address_space *mapping)
 {
-	return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
-		test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
+	return mapping_max_folio_order(mapping) > 0;
 }
 
 static inline int filemap_nr_thps(struct address_space *mapping)
-- 
2.39.2