fs/buffer: add folio_set_bh helper

Patch series "convert create_page_buffers to folio_create_buffers".

One of the first kernel panic we hit when we try to increase the block
size > 4k is inside create_page_buffers()[1].  Even though buffer.c
function do not support large folios (folios > PAGE_SIZE) at the moment,
these changes are required when we want to remove that constraint.


This patch (of 4):

The folio version of set_bh_page().  This is required to convert
create_page_buffers() to folio_create_buffers() later in the series.

Link: https://lkml.kernel.org/r/20230417123618.22094-1-p.raghav@samsung.com
Link: https://lkml.kernel.org/r/20230417123618.22094-2-p.raghav@samsung.comSigned-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -1485,6 +1485,21 @@ void set_bh_page(struct buffer_head *bh,
 }
 EXPORT_SYMBOL(set_bh_page);
 
+void folio_set_bh(struct buffer_head *bh, struct folio *folio,
+		  unsigned long offset)
+{
+	bh->b_folio = folio;
+	BUG_ON(offset >= folio_size(folio));
+	if (folio_test_highmem(folio))
+		/*
+		 * This catches illegal uses and preserves the offset:
+		 */
+		bh->b_data = (char *)(0 + offset);
+	else
+		bh->b_data = folio_address(folio) + offset;
+}
+EXPORT_SYMBOL(folio_set_bh);
+
 /*
  * Called when truncating a buffer on a page completely.
  */

include/linux/buffer_head.h

@@ -196,6 +196,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh);
 void touch_buffer(struct buffer_head *bh);
 void set_bh_page(struct buffer_head *bh,
 		struct page *page, unsigned long offset);
+void folio_set_bh(struct buffer_head *bh, struct folio *folio,
+		  unsigned long offset);
 bool try_to_free_buffers(struct folio *);
 struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 		bool retry);