fs: reduce stack usage in do_mpage_readpage

Some architectures support a very large PAGE_SIZE, so instead of the 8
pointers we see with a 4kB PAGE_SIZE, we can see 128 pointers with 64kB or
so many on Hexagon that it trips compiler warnings about exceeding stack
frame size.

All we're doing with this array is checking for block contiguity, which we
can as well do by remembering the address of the first block in the page
and checking this block is at the appropriate offset from that address.

Link: https://lkml.kernel.org/r/20231215200245.748418-5-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/mpage.c

@@ -166,7 +166,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 	sector_t block_in_file;
 	sector_t last_block;
 	sector_t last_block_in_file;
-	sector_t blocks[MAX_BUF_PER_PAGE];
+	sector_t first_block;
 	unsigned page_block;
 	unsigned first_hole = blocks_per_page;
 	struct block_device *bdev = NULL;
@@ -205,6 +205,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 		unsigned map_offset = block_in_file - args->first_logical_block;
 		unsigned last = nblocks - map_offset;
 
+		first_block = map_bh->b_blocknr + map_offset;
 		for (relative_block = 0; ; relative_block++) {
 			if (relative_block == last) {
 				clear_buffer_mapped(map_bh);
@@ -212,8 +213,6 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 			}
 			if (page_block == blocks_per_page)
 				break;
-			blocks[page_block] = map_bh->b_blocknr + map_offset +
-						relative_block;
 			page_block++;
 			block_in_file++;
 		}
@@ -259,7 +258,9 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 			goto confused;		/* hole -> non-hole */
 
 		/* Contiguous blocks? */
-		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
+		if (!page_block)
+			first_block = map_bh->b_blocknr;
+		else if (first_block + page_block != map_bh->b_blocknr)
 			goto confused;
 		nblocks = map_bh->b_size >> blkbits;
 		for (relative_block = 0; ; relative_block++) {
@@ -268,7 +269,6 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 				break;
 			} else if (page_block == blocks_per_page)
 				break;
-			blocks[page_block] = map_bh->b_blocknr+relative_block;
 			page_block++;
 			block_in_file++;
 		}
@@ -289,7 +289,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 	/*
 	 * This folio will go to BIO.  Do we need to send this BIO off first?
 	 */
-	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
+	if (args->bio && (args->last_block_in_bio != first_block - 1))
 		args->bio = mpage_bio_submit_read(args->bio);
 
 alloc_new:
@@ -298,7 +298,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 				      gfp);
 		if (args->bio == NULL)
 			goto confused;
-		args->bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
+		args->bio->bi_iter.bi_sector = first_block << (blkbits - 9);
 	}
 
 	length = first_hole << blkbits;
@@ -313,7 +313,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 	    (first_hole != blocks_per_page))
 		args->bio = mpage_bio_submit_read(args->bio);
 	else
-		args->last_block_in_bio = blocks[blocks_per_page - 1];
+		args->last_block_in_bio = first_block + blocks_per_page - 1;
 out:
 	return args->bio;