[PATCH RESEND v3 6/9] 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 | 18 ++++++++++++++++++
 mm/truncate.c           |  5 ++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b94c2e8ee918..29accb5d93b8 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);
@@ -531,6 +543,12 @@ static inline int split_folio_to_list(struct folio *folio, struct list_head *lis
 	return 0;
 }
 
+static inline int split_folio_at(struct folio *folio, struct page *page,
+		struct list_head *list)
+{
+	return 0;
+}
+
 static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
 #define split_huge_pmd(__vma, __pmd, __address)	\
 	do { } while (0)
diff --git a/mm/truncate.c b/mm/truncate.c
index 7c304d2f0052..9f33d6821748 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -178,6 +178,7 @@ 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;
@@ -207,7 +208,9 @@ 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