[PATCH v2 6/7] buffer: Handle large folios in __block_write_begin_int()

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

When __block_write_begin_int() was converted to support folios, we
did not expect large folios to be passed to it.  With the current
work to support large block size storage devices, this will no longer
be true so change the checks on 'from' and 'to' to be related to the
size of the folio instead of PAGE_SIZE.  Also remove an assumption that
the block size is smaller than PAGE_SIZE.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
 fs/buffer.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index faf1916200c2..ef444ab53a9b 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2075,27 +2075,24 @@ iomap_to_bh(struct inode *inode, sector_t block, struct buffer_head *bh,
 int __block_write_begin_int(struct folio *folio, loff_t pos, unsigned len,
 		get_block_t *get_block, const struct iomap *iomap)
 {
-	unsigned from = pos & (PAGE_SIZE - 1);
-	unsigned to = from + len;
+	size_t from = offset_in_folio(folio, pos);
+	size_t to = from + len;
 	struct inode *inode = folio->mapping->host;
-	unsigned block_start, block_end;
+	size_t block_start, block_end;
 	sector_t block;
 	int err = 0;
-	unsigned blocksize, bbits;
+	size_t blocksize;
 	struct buffer_head *bh, *head, *wait[2], **wait_bh=wait;
 
 	BUG_ON(!folio_test_locked(folio));
-	BUG_ON(from > PAGE_SIZE);
-	BUG_ON(to > PAGE_SIZE);
+	BUG_ON(to > folio_size(folio));
 	BUG_ON(from > to);
 
 	head = folio_create_buffers(folio, inode, 0);
 	blocksize = head->b_size;
-	bbits = block_size_bits(blocksize);
-
-	block = (sector_t)folio->index << (PAGE_SHIFT - bbits);
+	block = div_u64(folio_pos(folio), blocksize);
 
-	for(bh = head, block_start = 0; bh != head || !block_start;
+	for (bh = head, block_start = 0; bh != head || !block_start;
 	    block++, block_start=block_end, bh = bh->b_this_page) {
 		block_end = block_start + blocksize;
 		if (block_end <= from || block_start >= to) {
-- 
2.42.0