Re: semantics of rhashtable and sysvipc

[Linus Torvalds]

On Thu, May 24, 2018 at 10:23 AM Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:

>          tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
> -       if (tbl == NULL)
> -               return -ENOMEM;
> +       if (unlikely(tbl == NULL)) {
> +               size = min(size, HASH_DEFAULT_SIZE) / 2;
> +
> +               tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
> +               if (tbl == NULL)
> +                       tbl = bucket_table_alloc_retry(ht, size,
GFP_KERNEL);
> +       }

This doesn't seem to be taking 'param->min_size' into account.

I'm not sure that matters, but right now, if you have nelem_hint set and a
min_size, the min_size is honored (if you have just min_size it's already
ignored because the rhashtable always starts with HASH_DEFAULT_SIZE). So I
could imagine that somebody uses it to guarantee something. The docs say
that "min_size" is the minimum size for *shrinking* not for initializing,
so I guess it's debatable.

Also, wouldn't it make sense to make this all be a while loop? Or are you
just depending on the knowledge that HASH_DEFAULT_SIZE / 2 is already
guaranteed to be so small that there's no point? A comment to that effect
would be good, perhaps.

                    Linus