[PATCH 2/2] mm/huge_memory: cap max length on debugfs file entry folio split

[Luis Chamberlain]

Don't allow to query beyond a mapped file's length. Since this is just
a debugfs interface allow userspace to be lazy and use a large value so
we can just use the entire file.

Without this we can end up wasting cycles looking for folios which
just don't exist for no good reason.

Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
---
 mm/huge_memory.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8386d24a163e..86a8c7b3b8dc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3535,7 +3535,7 @@ static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start,
 	struct file *candidate;
 	struct address_space *mapping;
 	int ret = -EINVAL;
-	pgoff_t index;
+	pgoff_t index, fsize;
 	int nr_pages = 1;
 	unsigned long total = 0, split = 0;
 
@@ -3547,11 +3547,14 @@ static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start,
 	if (IS_ERR(candidate))
 		goto out;
 
+	mapping = candidate->f_mapping;
+	fsize = i_size_read(mapping->host);
+	if (off_end > fsize)
+		off_end = fsize;
+
 	pr_debug("split file-backed THPs in file: %s, page offset: [0x%lx - 0x%lx]\n",
 		 file_path, off_start, off_end);
 
-	mapping = candidate->f_mapping;
-
 	for (index = off_start; index < off_end; index += nr_pages) {
 		struct folio *folio = filemap_get_folio(mapping, index);
 
-- 
2.43.0