6.18-stable review patch. If anyone has any objections, please let me know.
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From: Yongjian Sun sunyongjian1@huawei.com
commit 3f7a79d05c692c7cfec70bf104b1b3c3d0ce6247 upstream.
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: c9de560ded61f ("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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/ext4/mballoc.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -783,6 +783,8 @@ static void __mb_check_buddy(struct ext4 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++)