ext4: fix incorrect group number assertion in mb_check_buddy

When the MB_CHECK_ASSERT macro is enabled, an assertion failure can
occur in __mb_check_buddy when checking preallocated blocks (pa) in
a block group:

Assertion failure in mb_free_blocks() : "groupnr == e4b->bd_group"

This happens when a pa at the very end of a block group (e.g.,
pa_pstart=32765, pa_len=3 in a group of 32768 blocks) becomes
exhausted - its pa_pstart is advanced by pa_len to 32768, which
lies in the next block group. If this exhausted pa (with pa_len == 0)
is still in the bb_prealloc_list during the buddy check, the assertion
incorrectly flags it as belonging to the wrong group. A possible
sequence is as follows:

ext4_mb_new_blocks
  ext4_mb_release_context
    pa->pa_pstart += EXT4_C2B(sbi, ac->ac_b_ex.fe_len)
    pa->pa_len -= ac->ac_b_ex.fe_len

	                 __mb_check_buddy
                           for each pa in group
                             ext4_get_group_no_and_offset
                             MB_CHECK_ASSERT(groupnr == e4b->bd_group)

To fix this, we modify the check to skip block group validation for
exhausted preallocations (where pa_len == 0). Such entries are in a
transitional state and will be removed from the list soon, so they
should not trigger an assertion. This change prevents the false
positive while maintaining the integrity of the checks for active
allocations.

Fixes: c9de560 ("ext4: Add multi block allocator for ext4")
Signed-off-by: Yongjian Sun <sunyongjian1@huawei.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20251106060614.631382-2-sunyongjian@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

Author: Yongjian Sun

fs/ext4/mballoc.c

@@ -768,6 +768,8 @@ static void __mb_check_buddy(struct ext4_buddy *e4b, char *file,
 		ext4_group_t groupnr;
 		struct ext4_prealloc_space *pa;
 		pa = list_entry(cur, struct ext4_prealloc_space, pa_group_list);
+		if (!pa->pa_len)
+			continue;
 		ext4_get_group_no_and_offset(sb, pa->pa_pstart, &groupnr, &k);
 		MB_CHECK_ASSERT(groupnr == e4b->bd_group);
 		for (i = 0; i < pa->pa_len; i++)