Re: [PATCH -v2] ext4: handle wraparound when searching for blocks for indirect mapped blocks

[Baokun Li <libaokun@linux.alibaba.com>]

On 3/26/26 12:58 PM, Theodore Ts'o wrote:
> Commit 4865c768b563 ("ext4: always allocate blocks only from groups
> inode can use") restricts what blocks will be allocated for indirect
> block based files to block numbers that fit within 32-bit block
> numbers.
>
> However, when using a review bot running on the latest Gemini LLM to
> check this commit when backporting into an LTS based kernel, it raised
> this concern:
>
>    If ac->ac_g_ex.fe_group is >= ngroups (for instance, if the goal
>    group was populated via stream allocation from s_mb_last_groups),
>    then start will be >= ngroups.
>
>    Does this allow allocating blocks beyond the 32-bit limit for
>    indirect block mapped files? The commit message mentions that
>    ext4_mb_scan_groups_linear() takes care to not select unsupported
>    groups. However, its loop uses group = *start, and the very first
>    iteration will call ext4_mb_scan_group() with this unsupported
>    group because next_linear_group() is only called at the end of the
>    iteration.
>
> After reviewing the code paths involved and considering the LLM
> review, I determined that this can happen when there is a file system
> where some files/directories are extent-mapped and others are
> indirect-block mapped.  To address this, add a safety clamp in
> ext4_mb_scan_groups().
>
> Fixes: 4865c768b563 ("ext4: always allocate blocks only from groups inode can use")
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> Cc: Jan Kara <jack@suse.cz>

Looks good to me.

Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>

> ---
> v2:
>   * Remove extra checks that were not needed once we add the clamp
>     in ext4_mb_scan_groups().
>
>  fs/ext4/mballoc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 20e9fdaf4301..b10db5d7545b 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1199,6 +1199,8 @@ static int ext4_mb_scan_groups(struct ext4_allocation_context *ac)
>  
>  	/* searching for the right group start from the goal value specified */
>  	start = ac->ac_g_ex.fe_group;
> +	if (start >= ngroups)
> +		start = 0;
>  	ac->ac_prefetch_grp = start;
>  	ac->ac_prefetch_nr = 0;
>