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SubjectRe: [PATCH 1/2] erofs: clean up z_erofs_extent_lookback
On Fri, 11 Mar 2022 02:27:42 +0800
Gao Xiang <hsiangkao@linux.alibaba.com> wrote:

> Avoid the unnecessary tail recursion since it can be converted into
> a loop directly in order to prevent potential stack overflow.
>
> It's a pretty straightforward conversion.
>
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---
> fs/erofs/zmap.c | 67 ++++++++++++++++++++++++-------------------------
> 1 file changed, 33 insertions(+), 34 deletions(-)
>
> diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
> index b4059b9c3bac..572f0b8151ba 100644
> --- a/fs/erofs/zmap.c
> +++ b/fs/erofs/zmap.c
> @@ -431,48 +431,47 @@ static int z_erofs_extent_lookback(struct z_erofs_maprecorder *m,
> unsigned int lookback_distance)
> {
> struct erofs_inode *const vi = EROFS_I(m->inode);
> - struct erofs_map_blocks *const map = m->map;
> const unsigned int lclusterbits = vi->z_logical_clusterbits;
> - unsigned long lcn = m->lcn;
> - int err;
>
> - if (lcn < lookback_distance) {
> - erofs_err(m->inode->i_sb,
> - "bogus lookback distance @ nid %llu", vi->nid);
> - DBG_BUGON(1);
> - return -EFSCORRUPTED;
> - }
> + while (m->lcn >= lookback_distance) {
> + unsigned long lcn = m->lcn - lookback_distance;
> + int err;

may better to declare variable 'lclusterbits' in loop just like 'err' usage?

>
> - /* load extent head logical cluster if needed */
> - lcn -= lookback_distance;
> - err = z_erofs_load_cluster_from_disk(m, lcn, false);
> - if (err)
> - return err;
> + /* load extent head logical cluster if needed */
> + err = z_erofs_load_cluster_from_disk(m, lcn, false);
> + if (err)
> + return err;
>
> - switch (m->type) {
> - case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
> - if (!m->delta[0]) {
> + switch (m->type) {
> + case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
> + if (!m->delta[0]) {
> + erofs_err(m->inode->i_sb,
> + "invalid lookback distance 0 @ nid %llu",
> + vi->nid);
> + DBG_BUGON(1);
> + return -EFSCORRUPTED;
> + }
> + lookback_distance = m->delta[0];
> + continue;
> + case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
> + case Z_EROFS_VLE_CLUSTER_TYPE_HEAD1:
> + case Z_EROFS_VLE_CLUSTER_TYPE_HEAD2:
> + m->headtype = m->type;
> + m->map->m_la = (lcn << lclusterbits) | m->clusterofs;
> + return 0;
> + default:
> erofs_err(m->inode->i_sb,
> - "invalid lookback distance 0 @ nid %llu",
> - vi->nid);
> + "unknown type %u @ lcn %lu of nid %llu",
> + m->type, lcn, vi->nid);
> DBG_BUGON(1);
> - return -EFSCORRUPTED;
> + return -EOPNOTSUPP;
> }
> - return z_erofs_extent_lookback(m, m->delta[0]);
> - case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
> - case Z_EROFS_VLE_CLUSTER_TYPE_HEAD1:
> - case Z_EROFS_VLE_CLUSTER_TYPE_HEAD2:
> - m->headtype = m->type;
> - map->m_la = (lcn << lclusterbits) | m->clusterofs;
> - break;
> - default:
> - erofs_err(m->inode->i_sb,
> - "unknown type %u @ lcn %lu of nid %llu",
> - m->type, lcn, vi->nid);
> - DBG_BUGON(1);
> - return -EOPNOTSUPP;
> }
> - return 0;
> +
> + erofs_err(m->inode->i_sb, "bogus lookback distance @ nid %llu",
> + vi->nid);
> + DBG_BUGON(1);
> + return -EFSCORRUPTED;
> }
>
> static int z_erofs_get_extent_compressedlen(struct z_erofs_maprecorder *m,

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