ext4: support large block size in ext4_mb_get_buddy_page_lock()

Currently, ext4_mb_get_buddy_page_lock() uses blocks_per_page to calculate
folio index and offset. However, when blocksize is larger than PAGE_SIZE,
blocks_per_page becomes zero, leading to a potential division-by-zero bug.

To support BS > PS, use bytes to compute folio index and offset within
folio to get rid of blocks_per_page.

Also, since ext4_mb_get_buddy_page_lock() already fully supports folio,
rename it to ext4_mb_get_buddy_folio_lock().

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Message-ID: <20251121090654.631996-13-libaokun@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

fs/ext4/mballoc.c

@@ -1527,50 +1527,52 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 }
 
 /*
- * Lock the buddy and bitmap pages. This make sure other parallel init_group
- * on the same buddy page doesn't happen whild holding the buddy page lock.
- * Return locked buddy and bitmap pages on e4b struct. If buddy and bitmap
- * are on the same page e4b->bd_buddy_folio is NULL and return value is 0.
+ * Lock the buddy and bitmap folios. This makes sure other parallel init_group
+ * on the same buddy folio doesn't happen while holding the buddy folio lock.
+ * Return locked buddy and bitmap folios on e4b struct. If buddy and bitmap
+ * are on the same folio e4b->bd_buddy_folio is NULL and return value is 0.
  */
-static int ext4_mb_get_buddy_page_lock(struct super_block *sb,
+static int ext4_mb_get_buddy_folio_lock(struct super_block *sb,
 		ext4_group_t group, struct ext4_buddy *e4b, gfp_t gfp)
 {
 	struct inode *inode = EXT4_SB(sb)->s_buddy_cache;
-	int block, pnum, poff;
-	int blocks_per_page;
+	int block, pnum;
 	struct folio *folio;
 
 	e4b->bd_buddy_folio = NULL;
 	e4b->bd_bitmap_folio = NULL;
 
-	blocks_per_page = PAGE_SIZE / sb->s_blocksize;
 	/*
 	 * the buddy cache inode stores the block bitmap
 	 * and buddy information in consecutive blocks.
 	 * So for each group we need two blocks.
 	 */
 	block = group * 2;
-	pnum = block / blocks_per_page;
-	poff = block % blocks_per_page;
+	pnum = EXT4_LBLK_TO_PG(inode, block);
 	folio = __filemap_get_folio(inode->i_mapping, pnum,
 			FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 	BUG_ON(folio->mapping != inode->i_mapping);
+	WARN_ON_ONCE(folio_size(folio) < sb->s_blocksize);
 	e4b->bd_bitmap_folio = folio;
-	e4b->bd_bitmap = folio_address(folio) + (poff * sb->s_blocksize);
+	e4b->bd_bitmap = folio_address(folio) +
+			 offset_in_folio(folio, EXT4_LBLK_TO_B(inode, block));
 
-	if (blocks_per_page >= 2) {
-		/* buddy and bitmap are on the same page */
+	block++;
+	pnum = EXT4_LBLK_TO_PG(inode, block);
+	if (folio_contains(folio, pnum)) {
+		/* buddy and bitmap are on the same folio */
 		return 0;
 	}
 
-	/* blocks_per_page == 1, hence we need another page for the buddy */
-	folio = __filemap_get_folio(inode->i_mapping, block + 1,
+	/* we need another folio for the buddy */
+	folio = __filemap_get_folio(inode->i_mapping, pnum,
 			FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 	BUG_ON(folio->mapping != inode->i_mapping);
+	WARN_ON_ONCE(folio_size(folio) < sb->s_blocksize);
 	e4b->bd_buddy_folio = folio;
 	return 0;
 }
@@ -1609,14 +1611,14 @@ int ext4_mb_init_group(struct super_block *sb, ext4_group_t group, gfp_t gfp)
 
 	/*
 	 * This ensures that we don't reinit the buddy cache
-	 * page which map to the group from which we are already
+	 * folio which map to the group from which we are already
 	 * allocating. If we are looking at the buddy cache we would
 	 * have taken a reference using ext4_mb_load_buddy and that
-	 * would have pinned buddy page to page cache.
-	 * The call to ext4_mb_get_buddy_page_lock will mark the
-	 * page accessed.
+	 * would have pinned buddy folio to page cache.
+	 * The call to ext4_mb_get_buddy_folio_lock will mark the
+	 * folio accessed.
 	 */
-	ret = ext4_mb_get_buddy_page_lock(sb, group, &e4b, gfp);
+	ret = ext4_mb_get_buddy_folio_lock(sb, group, &e4b, gfp);
 	if (ret || !EXT4_MB_GRP_NEED_INIT(this_grp)) {
 		/*
 		 * somebody initialized the group