Re: [PATCH v12 12/14] mm: multi-gen LRU: debugfs interface

[Yu Zhao <yuzhao@google.com>]

On Wed, Jun 22, 2022 at 3:16 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:

> > +static ssize_t lru_gen_seq_write(struct file *file, const char __user *src,
> > +                              size_t len, loff_t *pos)
> > +{
> > +     void *buf;
> > +     char *cur, *next;
> > +     unsigned int flags;
> > +     struct blk_plug plug;
> > +     int err = -EINVAL;
> > +     struct scan_control sc = {
> > +             .may_writepage = true,
> > +             .may_unmap = true,
> > +             .may_swap = true,
> > +             .reclaim_idx = MAX_NR_ZONES - 1,
> > +             .gfp_mask = GFP_KERNEL,
> > +     };
> > +
> > +     buf = kvmalloc(len + 1, GFP_KERNEL);
> > +     if (!buf)
> > +             return -ENOMEM;
> > +
> > +     if (copy_from_user(buf, src, len)) {
> > +             kvfree(buf);
> > +             return -EFAULT;
> > +     }
> > +
> > +     if (!set_mm_walk(NULL)) {
>
> The current->reclaim_state will be dereferenced in set_mm_walk(), so
> calling set_mm_walk() before set_task_reclaim_state(current,
> &sc.reclaim_state) will cause panic:
>
> [ 1861.154916] BUG: kernel NULL pointer dereference, address:
> 0000000000000008

Thanks.

Apparently I shot myself in the foot by one of the nits between v11 and v12.

> > +             kvfree(buf);
> > +             return -ENOMEM;
> > +     }
> > +
> > +     set_task_reclaim_state(current, &sc.reclaim_state);
> > +     flags = memalloc_noreclaim_save();
> > +     blk_start_plug(&plug);
> > +
> > +     next = buf;
> > +     next[len] = '\0';
> > +
> > +     while ((cur = strsep(&next, ",;\n"))) {
> > +             int n;
> > +             int end;
> > +             char cmd;
> > +             unsigned int memcg_id;
> > +             unsigned int nid;
> > +             unsigned long seq;
> > +             unsigned int swappiness = -1;
> > +             unsigned long opt = -1;
> > +
> > +             cur = skip_spaces(cur);
> > +             if (!*cur)
> > +                     continue;
> > +
> > +             n = sscanf(cur, "%c %u %u %lu %n %u %n %lu %n", &cmd, &memcg_id, &nid,
> > +                        &seq, &end, &swappiness, &end, &opt, &end);
> > +             if (n < 4 || cur[end]) {
> > +                     err = -EINVAL;
> > +                     break;
> > +             }
> > +
> > +             err = run_cmd(cmd, memcg_id, nid, seq, &sc, swappiness, opt);
> > +             if (err)
> > +                     break;
> > +     }
> > +
> > +     blk_finish_plug(&plug);
> > +     memalloc_noreclaim_restore(flags);
> > +     set_task_reclaim_state(current, NULL);
> > +
> > +     clear_mm_walk();
>
> Ditto, we can't call clear_mm_walk() after
> set_task_reclaim_state(current, NULL).
>
> Maybe it can be modified as follows:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2422edc786eb..552e6ae5243e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5569,12 +5569,12 @@ static ssize_t lru_gen_seq_write(struct file
> *file, const char __user *src,
>                  return -EFAULT;
>          }
>
> +       set_task_reclaim_state(current, &sc.reclaim_state);
>          if (!set_mm_walk(NULL)) {
>                  kvfree(buf);
>                  return -ENOMEM;
>          }
>
> -       set_task_reclaim_state(current, &sc.reclaim_state);

We need a `goto` because otherwise we leave a dangling
`current->reclaim_state`. (I swear I had one.)