[RFC v2 1/3] filemap: set max order to be min order if THP is disabled

[Pankaj Raghav <p.raghav@samsung.com>]

Large folios in the page cache depend on the splitting infrastructure from
THP. To remove the dependency between large folios and
CONFIG_TRANSPARENT_HUGEPAGE, set the min order == max order if THP is
disabled. This will make sure the splitting code will not be required
when THP is disabled, therefore, removing the dependency between large
folios and THP.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 include/linux/pagemap.h | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 09b581c1d878..1bb0d4432d4b 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -397,9 +397,7 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
  */
 static inline size_t mapping_max_folio_size_supported(void)
 {
-	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		return 1U << (PAGE_SHIFT + MAX_PAGECACHE_ORDER);
-	return PAGE_SIZE;
+	return 1U << (PAGE_SHIFT + MAX_PAGECACHE_ORDER);
 }
 
 /*
@@ -422,16 +420,17 @@ static inline void mapping_set_folio_order_range(struct address_space *mapping,
 						 unsigned int min,
 						 unsigned int max)
 {
-	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		return;
-
 	if (min > MAX_PAGECACHE_ORDER)
 		min = MAX_PAGECACHE_ORDER;
 
 	if (max > MAX_PAGECACHE_ORDER)
 		max = MAX_PAGECACHE_ORDER;
 
-	if (max < min)
+	/* Large folios depend on THP infrastructure for splitting.
+	 * If THP is disabled, we cap the max order to min order to avoid
+	 * splitting the folios.
+	 */
+	if ((max < min) || !IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
 		max = min;
 
 	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
@@ -463,16 +462,12 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
 static inline unsigned int
 mapping_max_folio_order(const struct address_space *mapping)
 {
-	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		return 0;
 	return (mapping->flags & AS_FOLIO_ORDER_MAX_MASK) >> AS_FOLIO_ORDER_MAX;
 }
 
 static inline unsigned int
 mapping_min_folio_order(const struct address_space *mapping)
 {
-	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		return 0;
 	return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
 }
 
-- 
2.50.1