linux-kernel - Re: semantics of rhashtable and sysvipc

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Message-ID: <CA+55aFwhkOnHngHpQ3wif5iy7PtOAVdjTcwce=kgBq0TCgmfJA@mail.gmail.com>
Date:   Thu, 24 May 2018 10:53:26 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        guillaume.knispel@...ersonicimagine.com,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: semantics of rhashtable and sysvipc

On Thu, May 24, 2018 at 10:23 AM Davidlohr Bueso <dave@...olabs.net> 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