ext4: support large block size in ext4_mb_init_cache()

Currently, ext4_mb_init_cache() uses blocks_per_page to calculate the
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.

Since we now have the folio, we know its exact size. This allows us to
convert {blocks, groups}_per_page to {blocks, groups}_per_folio, thus
supporting block sizes greater than page size.

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: <20251121090654.631996-14-libaokun@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

fs/ext4/mballoc.c

@@ -1346,26 +1346,25 @@ static void mb_regenerate_buddy(struct ext4_buddy *e4b)
  * block bitmap and buddy information. The information are
  * stored in the inode as
  *
- * {                        page                        }
+ * {                        folio                        }
  * [ group 0 bitmap][ group 0 buddy] [group 1][ group 1]...
  *
  *
  * one block each for bitmap and buddy information.
- * So for each group we take up 2 blocks. A page can
- * contain blocks_per_page (PAGE_SIZE / blocksize)  blocks.
- * So it can have information regarding groups_per_page which
- * is blocks_per_page/2
+ * So for each group we take up 2 blocks. A folio can
+ * contain blocks_per_folio (folio_size / blocksize)  blocks.
+ * So it can have information regarding groups_per_folio which
+ * is blocks_per_folio/2
  *
  * Locking note:  This routine takes the block group lock of all groups
- * for this page; do not hold this lock when calling this routine!
+ * for this folio; do not hold this lock when calling this routine!
  */
-
 static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 {
 	ext4_group_t ngroups;
 	unsigned int blocksize;
-	int blocks_per_page;
-	int groups_per_page;
+	int blocks_per_folio;
+	int groups_per_folio;
 	int err = 0;
 	int i;
 	ext4_group_t first_group, group;
@@ -1382,35 +1381,32 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 	sb = inode->i_sb;
 	ngroups = ext4_get_groups_count(sb);
 	blocksize = i_blocksize(inode);
-	blocks_per_page = PAGE_SIZE / blocksize;
+	blocks_per_folio = folio_size(folio) / blocksize;
+	WARN_ON_ONCE(!blocks_per_folio);
+	groups_per_folio = DIV_ROUND_UP(blocks_per_folio, 2);
 
 	mb_debug(sb, "init folio %lu\n", folio->index);
 
-	groups_per_page = blocks_per_page >> 1;
-	if (groups_per_page == 0)
-		groups_per_page = 1;
-
 	/* allocate buffer_heads to read bitmaps */
-	if (groups_per_page > 1) {
-		i = sizeof(struct buffer_head *) * groups_per_page;
+	if (groups_per_folio > 1) {
+		i = sizeof(struct buffer_head *) * groups_per_folio;
 		bh = kzalloc(i, gfp);
 		if (bh == NULL)
 			return -ENOMEM;
 	} else
 		bh = &bhs;
 
-	first_group = folio->index * blocks_per_page / 2;
-
 	/* read all groups the folio covers into the cache */
-	for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
+	first_group = EXT4_PG_TO_LBLK(inode, folio->index) / 2;
+	for (i = 0, group = first_group; i < groups_per_folio; i++, group++) {
 		if (group >= ngroups)
 			break;
 
 		grinfo = ext4_get_group_info(sb, group);
 		if (!grinfo)
 			continue;
 		/*
-		 * If page is uptodate then we came here after online resize
+		 * If folio is uptodate then we came here after online resize
 		 * which added some new uninitialized group info structs, so
 		 * we must skip all initialized uptodate buddies on the folio,
 		 * which may be currently in use by an allocating task.
@@ -1430,7 +1426,7 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 	}
 
 	/* wait for I/O completion */
-	for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
+	for (i = 0, group = first_group; i < groups_per_folio; i++, group++) {
 		int err2;
 
 		if (!bh[i])
@@ -1440,8 +1436,8 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 			err = err2;
 	}
 
-	first_block = folio->index * blocks_per_page;
-	for (i = 0; i < blocks_per_page; i++) {
+	first_block = EXT4_PG_TO_LBLK(inode, folio->index);
+	for (i = 0; i < blocks_per_folio; i++) {
 		group = (first_block + i) >> 1;
 		if (group >= ngroups)
 			break;
@@ -1518,7 +1514,7 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
 
 out:
 	if (bh) {
-		for (i = 0; i < groups_per_page; i++)
+		for (i = 0; i < groups_per_folio; i++)
 			brelse(bh[i]);
 		if (bh != &bhs)
 			kfree(bh);