[PATCH 03/14] fs: Reduce stack usage in __mpage_writepage

[Matthew Wilcox (Oracle)]

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.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 fs/mpage.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/fs/mpage.c b/fs/mpage.c
index 630f4a7c7d03..84b02098e7a5 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -466,7 +466,7 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
 	sector_t last_block;
 	sector_t block_in_file;
-	sector_t blocks[MAX_BUF_PER_PAGE];
+	sector_t first_block;
 	unsigned page_block;
 	unsigned first_unmapped = blocks_per_page;
 	struct block_device *bdev = NULL;
@@ -504,10 +504,12 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 				goto confused;
 			if (page_block) {
-				if (bh->b_blocknr != blocks[page_block-1] + 1)
+				if (bh->b_blocknr != first_block + page_block)
 					goto confused;
+			} else {
+				first_block = bh->b_blocknr;
 			}
-			blocks[page_block++] = bh->b_blocknr;
+			page_block++;
 			boundary = buffer_boundary(bh);
 			if (boundary) {
 				boundary_block = bh->b_blocknr;
@@ -556,10 +558,12 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 			boundary_bdev = map_bh.b_bdev;
 		}
 		if (page_block) {
-			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
+			if (map_bh.b_blocknr != first_block + page_block)
 				goto confused;
+		} else {
+			first_block = map_bh.b_blocknr;
 		}
-		blocks[page_block++] = map_bh.b_blocknr;
+		page_block++;
 		boundary = buffer_boundary(&map_bh);
 		bdev = map_bh.b_bdev;
 		if (block_in_file == last_block)
@@ -591,7 +595,7 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 	/*
 	 * This page will go to BIO.  Do we need to send this BIO off first?
 	 */
-	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
+	if (bio && mpd->last_block_in_bio != first_block - 1)
 		bio = mpage_bio_submit_write(bio);
 
 alloc_new:
@@ -599,7 +603,7 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 		bio = bio_alloc(bdev, BIO_MAX_VECS,
 				REQ_OP_WRITE | wbc_to_write_flags(wbc),
 				GFP_NOFS);
-		bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
+		bio->bi_iter.bi_sector = first_block << (blkbits - 9);
 		wbc_init_bio(wbc, bio);
 	}
 
@@ -627,7 +631,7 @@ static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
 					boundary_block, 1 << blkbits);
 		}
 	} else {
-		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
+		mpd->last_block_in_bio = first_block + blocks_per_page - 1;
 	}
 	goto out;
 
-- 
2.42.0