mm/huge_memory: add folio_split() to debugfs testing interface

This allows to test folio_split() by specifying an additional in folio
page offset parameter to split_huge_page debugfs interface.

Link: https://lkml.kernel.org/r/20250307174001.242794-7-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -4331,7 +4331,8 @@ static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma)
 }
 
 static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
-				unsigned long vaddr_end, unsigned int new_order)
+				unsigned long vaddr_end, unsigned int new_order,
+				long in_folio_offset)
 {
 	int ret = 0;
 	struct task_struct *task;
@@ -4415,8 +4416,16 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		if (!folio_test_anon(folio) && folio->mapping != mapping)
 			goto unlock;
 
-		if (!split_folio_to_order(folio, target_order))
-			split++;
+		if (in_folio_offset < 0 ||
+		    in_folio_offset >= folio_nr_pages(folio)) {
+			if (!split_folio_to_order(folio, target_order))
+				split++;
+		} else {
+			struct page *split_at = folio_page(folio,
+							   in_folio_offset);
+			if (!folio_split(folio, target_order, split_at, NULL))
+				split++;
+		}
 
 unlock:
 
@@ -4439,7 +4448,8 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 }
 
 static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start,
-				pgoff_t off_end, unsigned int new_order)
+				pgoff_t off_end, unsigned int new_order,
+				long in_folio_offset)
 {
 	struct filename *file;
 	struct file *candidate;
@@ -4488,8 +4498,15 @@ static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start,
 		if (folio->mapping != mapping)
 			goto unlock;
 
-		if (!split_folio_to_order(folio, target_order))
-			split++;
+		if (in_folio_offset < 0 || in_folio_offset >= nr_pages) {
+			if (!split_folio_to_order(folio, target_order))
+				split++;
+		} else {
+			struct page *split_at = folio_page(folio,
+							   in_folio_offset);
+			if (!folio_split(folio, target_order, split_at, NULL))
+				split++;
+		}
 
 unlock:
 		folio_unlock(folio);
@@ -4522,6 +4539,7 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
 	int pid;
 	unsigned long vaddr_start, vaddr_end;
 	unsigned int new_order = 0;
+	long in_folio_offset = -1;
 
 	ret = mutex_lock_interruptible(&split_debug_mutex);
 	if (ret)
@@ -4550,30 +4568,33 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
 			goto out;
 		}
 
-		ret = sscanf(tok_buf, "0x%lx,0x%lx,%d", &off_start,
-			    &off_end, &new_order);
-		if (ret != 2 && ret != 3) {
+		ret = sscanf(tok_buf, "0x%lx,0x%lx,%d,%ld", &off_start, &off_end,
+				&new_order, &in_folio_offset);
+		if (ret != 2 && ret != 3 && ret != 4) {
 			ret = -EINVAL;
 			goto out;
 		}
-		ret = split_huge_pages_in_file(file_path, off_start, off_end, new_order);
+		ret = split_huge_pages_in_file(file_path, off_start, off_end,
+				new_order, in_folio_offset);
 		if (!ret)
 			ret = input_len;
 
 		goto out;
 	}
 
-	ret = sscanf(input_buf, "%d,0x%lx,0x%lx,%d", &pid, &vaddr_start, &vaddr_end, &new_order);
+	ret = sscanf(input_buf, "%d,0x%lx,0x%lx,%d,%ld", &pid, &vaddr_start,
+			&vaddr_end, &new_order, &in_folio_offset);
 	if (ret == 1 && pid == 1) {
 		split_huge_pages_all();
 		ret = strlen(input_buf);
 		goto out;
-	} else if (ret != 3 && ret != 4) {
+	} else if (ret != 3 && ret != 4 && ret != 5) {
 		ret = -EINVAL;
 		goto out;
 	}
 
-	ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end, new_order);
+	ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end, new_order,
+			in_folio_offset);
 	if (!ret)
 		ret = strlen(input_buf);
 out: