btrfs: add and use readahead_batch_length

Implement readahead_batch_length() to determine the number of bytes in
the current batch of readahead pages and use it in btrfs. Also use the
readahead_pos to get the offset.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4650,10 +4650,8 @@ void extent_readahead(struct readahead_control *rac)
 	int nr;
 
 	while ((nr = readahead_page_batch(rac, pagepool))) {
-		u64 contig_start = page_offset(pagepool[0]);
-		u64 contig_end = page_offset(pagepool[nr - 1]) + PAGE_SIZE - 1;
-
-		ASSERT(contig_start + nr * PAGE_SIZE - 1 == contig_end);
+		u64 contig_start = readahead_pos(rac);
+		u64 contig_end = contig_start + readahead_batch_length(rac) - 1;
 
 		contiguous_readpages(pagepool, nr, contig_start, contig_end,
 				&em_cached, &bio, &bio_flags, &prev_em_start);

include/linux/pagemap.h

@@ -981,6 +981,15 @@ static inline unsigned int readahead_count(struct readahead_control *rac)
 	return rac->_nr_pages;
 }
 
+/**
+ * readahead_batch_length - The number of bytes in the current batch.
+ * @rac: The readahead request.
+ */
+static inline loff_t readahead_batch_length(struct readahead_control *rac)
+{
+	return rac->_batch_count * PAGE_SIZE;
+}
+
 static inline unsigned long dir_pages(struct inode *inode)
 {
 	return (unsigned long)(inode->i_size + PAGE_SIZE - 1) >>