mm/filemap: fix page end in filemap_get_read_batch

I was running traces of the read code against an RAID storage system to
understand why read requests were being misaligned against the underlying
RAID strips.  I found that the page end offset calculation in
filemap_get_read_batch() was off by one.

When a read is submitted with end offset 1048575, then it calculates the
end page for read of 256 when it should be 255.  "last_index" is the index
of the page beyond the end of the read and it should be skipped when get a
batch of pages for read in @filemap_get_read_batch().

The below simple patch fixes the problem.  This code was introduced in
kernel 5.12.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: cbd59c4 ("mm/filemap: use head pages in generic_file_buffered_read")
Signed-off-by: Qian Yingjin <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: yingjinqian

mm/filemap.c

@@ -2588,18 +2588,19 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 	struct folio *folio;
 	int err = 0;
 
+	/* "last_index" is the index of the page beyond the end of the read */
 	last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
 retry:
 	if (fatal_signal_pending(current))
 		return -EINTR;
 
-	filemap_get_read_batch(mapping, index, last_index, fbatch);
+	filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & IOCB_NOIO)
 			return -EAGAIN;
 		page_cache_sync_readahead(mapping, ra, filp, index,
 				last_index - index);
-		filemap_get_read_batch(mapping, index, last_index, fbatch);
+		filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	}
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))