linux-kernel - Re: [PATCH 5.4 175/244] inet: use bigger hash table for IP ID generation

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Date:   Wed, 16 Jun 2021 12:29:06 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Amit Klein' <aksecurity@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>, Willy Tarreau <w@....eu>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.4 175/244] inet: use bigger hash table for IP ID
 generation

On Wed, Jun 16, 2021 at 10:19:15AM +0000, David Laight wrote:
> From: Amit Klein
> > Sent: 16 June 2021 10:17
> ...
> > -#define IP_IDENTS_SZ 2048u
> > -
> > +/* Hash tables of size 2048..262144 depending on RAM size.
> > + * Each bucket uses 8 bytes.
> > + */
> > +static u32 ip_idents_mask __read_mostly;
> ...
> > +    /* For modern hosts, this will use 2 MB of memory */
> > +    idents_hash = alloc_large_system_hash("IP idents",
> > +                          sizeof(*ip_idents) + sizeof(*ip_tstamps),
> > +                          0,
> > +                          16, /* one bucket per 64 KB */
> > +                          HASH_ZERO,
> > +                          NULL,
> > +                          &ip_idents_mask,
> > +                          2048,
> > +                          256*1024);
> > +
> 
> Can someone explain why this is a good idea for a 'normal' system?
> 
> Why should my desktop system 'waste' 2MB of memory on a massive
> hash table that I don't need.
> It might be needed by systems than handle massive numbers
> of concurrent connections - but that isn't 'most systems'.
> 
> Surely it would be better to detect when the number of entries
> is comparable to the table size and then resize the table.

Patches always gladly accepted.