[PATCH v1 3/3] mm/truncate: use folio_split() for truncate operation.

[Zi Yan <ziy@nvidia.com>]

Instead of splitting the large folio uniformly during truncation, use
buddy allocator like split at the start of truncation range to minimize
the number of resulting folios.

For example, to truncate a order-4 folio
[0, 1, 2, 3, 4, 5, ..., 15]
between [3, 10] (inclusive), folio_split() splits the folio to
[0,1], [2], [3], [4..7], [8..15] and [3], [4..7] can be dropped and
[8..15] is kept with zeros in [8..10].

It is possible to further do a folio_split() at 10, so more resulting
folios can be dropped. But it is left as future possible optimization
if needed.

Another possible optimization is to make folio_split() to split a folio
based on a given range, like [3..10] above. But that complicates
folio_split(), so it will investigated when necessary.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/huge_mm.h | 12 ++++++++++++
 mm/huge_memory.c        |  2 +-
 mm/truncate.c           |  5 ++++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b94c2e8ee918..8048500e7bc2 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -339,6 +339,18 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		unsigned int new_order);
 int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
+int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
+		struct list_head *list);
+static inline int split_folio_at(struct folio *folio, struct page *page,
+		struct list_head *list)
+{
+	int ret = min_order_for_split(folio);
+
+	if (ret < 0)
+		return ret;
+
+	return folio_split(folio, ret, page, list);
+}
 static inline int split_huge_page(struct page *page)
 {
 	struct folio *folio = page_folio(page);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4ccd23473e2b..a688b73fa793 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3803,7 +3803,7 @@ int split_folio_to_list(struct folio *folio, struct list_head *list)
  *
  * After split, folio is left locked for caller.
  */
-static int folio_split(struct folio *folio, unsigned int new_order,
+int folio_split(struct folio *folio, unsigned int new_order,
 		struct page *page, struct list_head *list)
 {
 	return __folio_split(folio, new_order, page, list, false);
diff --git a/mm/truncate.c b/mm/truncate.c
index e5151703ba04..dbd81c21b460 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -179,6 +179,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 {
 	loff_t pos = folio_pos(folio);
 	unsigned int offset, length;
+	long in_folio_offset;
 
 	if (pos < start)
 		offset = start - pos;
@@ -208,7 +209,9 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 		folio_invalidate(folio, offset, length);
 	if (!folio_test_large(folio))
 		return true;
-	if (split_folio(folio) == 0)
+
+	in_folio_offset = PAGE_ALIGN_DOWN(offset) / PAGE_SIZE;
+	if (split_folio_at(folio, folio_page(folio, in_folio_offset), NULL) == 0)
 		return true;
 	if (folio_test_dirty(folio))
 		return false;
-- 
2.45.2