netdev - Re: xfrm_state locking regression...

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Date:	Tue, 23 Sep 2008 20:56:15 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Timo Teräs <timo.teras@....fi>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: xfrm_state locking regression...

On Tue, Sep 23, 2008 at 03:25:41PM +0300, Timo Teräs wrote:
>
> There can be a lot of xfrm_state structs. As in thousands. So it
> will take more memory then. hlist would be enough, so it'd be a
> 4/8 bytes addition. Single pointer would not be enough as we can
> have multiple walks on the same entry.

Right, we need doubly linked lists in the walkers to ease unlinking.
But only a single pointer (i.e., an hlist) is needed in xfrm_state.

> I was thinking about the same. That's why I wasn't so sure if this is
> better. In practice there aren't many walks active. Maybe we limit
> the maximum simultaneous walks?

That sounds like a hack :)

> But in real life, there is only a one or two walkers if any active.
> So I would not consider a global update too expensive. But if you think
> it might become a problem, I'd add an hlist to xfrm_state of walkers
> referencing that entry.

Well in real life dumps don't tend to get suspended either :)
In any case, a suspended dump is only going to pin down some
memory, while a very long list walk may kill the box.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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