[PATCH v14 28/39] mm/filemap: Add folio_mkwrite_check_truncate()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

This is the folio equivalent of page_mkwrite_check_truncate().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/pagemap.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 412db88b8d0c..18c06c3e42c3 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1121,6 +1121,34 @@ static inline unsigned long dir_pages(struct inode *inode)
 			       PAGE_SHIFT;
 }
 
+/**
+ * folio_mkwrite_check_truncate - check if folio was truncated
+ * @folio: the folio to check
+ * @inode: the inode to check the folio against
+ *
+ * Return: the number of bytes in the folio up to EOF,
+ * or -EFAULT if the folio was truncated.
+ */
+static inline ssize_t folio_mkwrite_check_truncate(struct folio *folio,
+					      struct inode *inode)
+{
+	loff_t size = i_size_read(inode);
+	pgoff_t index = size >> PAGE_SHIFT;
+	size_t offset = offset_in_folio(folio, size);
+
+	if (!folio->mapping)
+		return -EFAULT;
+
+	/* folio is wholly inside EOF */
+	if (folio_next_index(folio) - 1 < index)
+		return folio_size(folio);
+	/* folio is wholly past EOF */
+	if (folio->index > index || !offset)
+		return -EFAULT;
+	/* folio is partially inside EOF */
+	return offset;
+}
+
 /**
  * page_mkwrite_check_truncate - check if page was truncated
  * @page: the page to check
-- 
2.30.2