ext4: remove array of buffer_heads from mext_page_mkuptodate()

Iterate the folio's list of buffer_heads twice instead of keeping
an array of pointers.  This solves a too-large-array-for-stack problem
on architectures with a ridiculoously large PAGE_SIZE and prepares
ext4 to support larger folios.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20240718223005.568869-3-willy@infradead.orgSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/move_extent.c

@@ -166,15 +166,14 @@ mext_folio_double_lock(struct inode *inode1, struct inode *inode2,
 	return 0;
 }
 
-/* Force page buffers uptodate w/o dropping page's lock */
-static int
-mext_page_mkuptodate(struct folio *folio, unsigned from, unsigned to)
+/* Force folio buffers uptodate w/o dropping folio's lock */
+static int mext_page_mkuptodate(struct folio *folio, size_t from, size_t to)
 {
 	struct inode *inode = folio->mapping->host;
 	sector_t block;
-	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+	struct buffer_head *bh, *head;
 	unsigned int blocksize, block_start, block_end;
-	int i, nr = 0;
+	int nr = 0;
 	bool partial = false;
 
 	BUG_ON(!folio_test_locked(folio));
@@ -215,20 +214,23 @@ mext_page_mkuptodate(struct folio *folio, unsigned from, unsigned to)
 			continue;
 		}
 		ext4_read_bh_nowait(bh, 0, NULL);
-		BUG_ON(nr >= MAX_BUF_PER_PAGE);
-		arr[nr++] = bh;
+		nr++;
 	}
 	/* No io required */
 	if (!nr)
 		goto out;
 
-	for (i = 0; i < nr; i++) {
-		bh = arr[i];
+	bh = head;
+	do {
+		if (bh_offset(bh) + blocksize <= from)
+			continue;
+		if (bh_offset(bh) > to)
+			break;
 		wait_on_buffer(bh);
 		if (buffer_uptodate(bh))
 			continue;
 		return -EIO;
-	}
+	} while ((bh = bh->b_this_page) != head);
 out:
 	if (!partial)
 		folio_mark_uptodate(folio);