Re: [PATCH 6/7] mm/filemap: allocate folios with mapping blocksize

[Matthew Wilcox <willy@infradead.org>]

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On Tue, Jun 20, 2023 at 08:57:48AM +1000, Dave Chinner wrote:
> What is needed is a field into the mapping that defines the
> folio order that all folios allocated for the page cache must be
> aligned/sized to to allow them to be inserted into the mapping.

Attached patch from December 2020 ;-)

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From 1aeee696f4d322af5f34544e39fc00006c399fb8 Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Tue, 15 Dec 2020 10:57:34 -0500
Subject: [PATCH] fs: Allow fine-grained control of folio sizes

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).

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/pagemap.h | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index cad81db32e61..9cbb8bdbaee7 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -198,9 +198,15 @@ 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
@@ -290,6 +296,29 @@ 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.
@@ -303,7 +332,12 @@ 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;
 }
 
 /*
@@ -312,8 +346,7 @@ 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)
-- 
2.35.1