ext4: support large block size in ext4_mb_load_buddy_gfp()

Currently, ext4_mb_load_buddy_gfp() 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.

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

Also, if buddy and bitmap land in the same folio, we get that folio’s ref
instead of looking it up again before updating the buddy.

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: <20251121090654.631996-12-libaokun@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 65335248825ce45c5c5ad13c9ae5ce0440cfdb51..38af89f5da566196b18984f7231a315023da6a34 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1659,17 +1659,15 @@ int ext4_mb_init_group(struct super_block *sb, ext4_group_t group, gfp_t gfp)
 
 /*
  * Locking note:  This routine calls ext4_mb_init_cache(), which takes the
- * block group lock of all groups for this page; do not hold the BG lock when
+ * block group lock of all groups for this folio; do not hold the BG lock when
  * calling this routine!
  */
 static noinline_for_stack int
 ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
                       struct ext4_buddy *e4b, gfp_t gfp)
 {
-       int blocks_per_page;
        int block;
        int pnum;
-       int poff;
        struct folio *folio;
        int ret;
        struct ext4_group_info *grp;
@@ -1679,7 +1677,6 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
        might_sleep();
        mb_debug(sb, "load group %u\n", group);
 
-       blocks_per_page = PAGE_SIZE / sb->s_blocksize;
        grp = ext4_get_group_info(sb, group);
        if (!grp)
                return -EFSCORRUPTED;
@@ -1707,8 +1704,7 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
         * 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);
 
        /* Avoid locking the folio in the fast path ... */
        folio = __filemap_get_folio(inode->i_mapping, pnum, FGP_ACCESSED, 0);
@@ -1740,7 +1736,8 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
                                        goto err;
                                }
                                mb_cmp_bitmaps(e4b, folio_address(folio) +
-                                              (poff * sb->s_blocksize));
+                                       offset_in_folio(folio,
+                                               EXT4_LBLK_TO_B(inode, block)));
                        }
                        folio_unlock(folio);
                }
@@ -1756,12 +1753,18 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
 
        /* Folios marked accessed already */
        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));
 
        block++;
-       pnum = block / blocks_per_page;
-       poff = block % blocks_per_page;
+       pnum = EXT4_LBLK_TO_PG(inode, block);
+       /* buddy and bitmap are on the same folio? */
+       if (folio_contains(folio, pnum)) {
+               folio_get(folio);
+               goto update_buddy;
+       }
 
+       /* we need another folio for the buddy */
        folio = __filemap_get_folio(inode->i_mapping, pnum, FGP_ACCESSED, 0);
        if (IS_ERR(folio) || !folio_test_uptodate(folio)) {
                if (!IS_ERR(folio))
@@ -1796,9 +1799,11 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
                goto err;
        }
 
+update_buddy:
        /* Folios marked accessed already */
        e4b->bd_buddy_folio = folio;
-       e4b->bd_buddy = folio_address(folio) + (poff * sb->s_blocksize);
+       e4b->bd_buddy = folio_address(folio) +
+                       offset_in_folio(folio, EXT4_LBLK_TO_B(inode, block));
 
        return 0;