buffer: fix more functions for block size > PAGE_SIZE

Both __block_write_full_folio() and block_read_full_folio() assumed that
block size <= PAGE_SIZE.  Replace the shift with a divide, which is
probably cheaper than first calculating the shift.  That lets us remove
block_size_bits() as these were the last callers.

Link: https://lkml.kernel.org/r/20231109210608.2252323-8-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -1742,19 +1742,6 @@ void clean_bdev_aliases(struct block_device *bdev, sector_t block, sector_t len)
 }
 EXPORT_SYMBOL(clean_bdev_aliases);
 
-/*
- * Size is a power-of-two in the range 512..PAGE_SIZE,
- * and the case we care about most is PAGE_SIZE.
- *
- * So this *could* possibly be written with those
- * constraints in mind (relevant mostly if some
- * architecture has a slow bit-scan instruction)
- */
-static inline int block_size_bits(unsigned int blocksize)
-{
-	return ilog2(blocksize);
-}
-
 static struct buffer_head *folio_create_buffers(struct folio *folio,
 						struct inode *inode,
 						unsigned int b_state)
@@ -1807,7 +1794,7 @@ int __block_write_full_folio(struct inode *inode, struct folio *folio,
 	sector_t block;
 	sector_t last_block;
 	struct buffer_head *bh, *head;
-	unsigned int blocksize, bbits;
+	size_t blocksize;
 	int nr_underway = 0;
 	blk_opf_t write_flags = wbc_to_write_flags(wbc);
 
@@ -1826,10 +1813,9 @@ int __block_write_full_folio(struct inode *inode, struct folio *folio,
 
 	bh = head;
 	blocksize = bh->b_size;
-	bbits = block_size_bits(blocksize);
 
-	block = (sector_t)folio->index << (PAGE_SHIFT - bbits);
-	last_block = (i_size_read(inode) - 1) >> bbits;
+	block = div_u64(folio_pos(folio), blocksize);
+	last_block = div_u64(i_size_read(inode) - 1, blocksize);
 
 	/*
 	 * Get all the dirty buffers mapped to disk addresses and
@@ -2355,7 +2341,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 	struct inode *inode = folio->mapping->host;
 	sector_t iblock, lblock;
 	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
-	unsigned int blocksize, bbits;
+	size_t blocksize;
 	int nr, i;
 	int fully_mapped = 1;
 	bool page_error = false;
@@ -2369,10 +2355,9 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 
 	head = folio_create_buffers(folio, inode, 0);
 	blocksize = head->b_size;
-	bbits = block_size_bits(blocksize);
 
-	iblock = (sector_t)folio->index << (PAGE_SHIFT - bbits);
-	lblock = (limit+blocksize-1) >> bbits;
+	iblock = div_u64(folio_pos(folio), blocksize);
+	lblock = div_u64(limit + blocksize - 1, blocksize);
 	bh = head;
 	nr = 0;
 	i = 0;