Re: [PATCH v2 4/9] exfat: add support for multi-cluster allocation

[Namjae Jeon]

On Thu, May 7, 2026 at 11:09 PM CharSyam <charsyam@xxxxxxxxx> wrote:
>
> hi, Namjae.
>
> The critical issue is that this hunk drops the hole-care walk in
>   exfat_map_cluster():
>
>                   *clu = new_clu.dir;
>   +               *count = new_clu.size;
>
>   -               inode->i_blocks +=
>   -                       exfat_cluster_to_sectors(sbi,
> num_to_be_allocated) >> 9;
>   -
>   -               /*
>   -                * Move *clu pointer along FAT chains (hole care) because the
>   -                * caller of this function expect *clu to be the last cluster.
>   -                * This only works when num_to_be_allocated >= 2,
>   -                * *clu = (the first cluster of the allocated chain) =>
>   -                * (the last cluster of ...)
>   -                */
>   -               if (exfat_cluster_walk(sb, clu, num_to_be_allocated
> - 1, ei->flags))
>   -                       return -EIO;
>   -               *count = 1;
>   +               inode->i_blocks += exfat_cluster_to_sectors(sbi,
> new_clu.size);
>
>   The removed comment is explicit: the caller expects *clu to be the
>   cluster at the requested clu_offset, not necessarily the first cluster
>   of the newly allocated chain. The old code walked
>   num_to_be_allocated - 1 steps so that *clu ended up at clu_offset.
>
>   With this change, *clu is left as new_clu.dir (the first newly
>   allocated cluster) and *count is set to new_clu.size. That is only
>   correct for the sequential-extension case where
>   clu_offset == num_clusters. For clu_offset > num_clusters, the caller
>   will compute the physical sector from the wrong cluster:
>
>         phys = exfat_cluster_to_sector(sbi, cluster) + sec_offset;
>
>   This also interacts badly with the new contig=true allocation mode.
>   If exfat_alloc_cluster() stops early at a non-contiguous boundary,
>   new_clu.size can be smaller than num_to_be_allocated, so the allocated
>   run may not even reach clu_offset while exfat_map_cluster() still
>   returns success.
>
>   Could you keep the old hole-care behavior and extend it to the
>   multi-cluster case? After allocation, the code should advance from
>   new_clu.dir to the cluster corresponding to clu_offset, set *count to
>   the remaining number of clusters from that point, and handle partial
>   contiguous allocation by either continuing allocation or unwinding it
>   before returning an error.
exfat_map_cluster() returns a contiguous cluster range to the caller :
a starting cluster number (*clu) and the number of contiguous
clusters(*count) from that point. num_to_be_allocated is only the
maximum number of clusters the caller would like to allocate. If
exfat_alloc_cluster() returns fewer clusters than requested, that is
not an error.

Thanks.