linux-kernel - Re: [PATCH 5/5] tun: vringfd xmit support.

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Message-Id: <20080418120634.986e30f1.akpm@linux-foundation.org>
Date:	Fri, 18 Apr 2008 12:06:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	netdev@...r.kernel.org, Max Krasnyansky <maxk@...lcomm.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] tun: vringfd xmit support.

On Sat, 19 Apr 2008 01:15:15 +1000 Rusty Russell <rusty@...tcorp.com.au> wrote:

> >
> > What is the maximum numbet of pages which an unpriviliged user can
> > concurrently pin with this code?
> 
> Since only root can open the tun device, it's currently OK.  The old code
> kmalloced and copied: is there some mm-fu reason why pinning userspace memory
> is worse?

We generally try to avoid it - it allows users to dos the box.  Although I
suspect that direct-io presently permits users to transiently pin an amount
of memory which is proportional to the number of disks upon which they can
open files.

> Subject: Export release_pages; nice undo for get_user_pages.
> 
> Andrew Morton suggests tun/tap use release_pages, but it's not
> exported.  It's not clear to me why this is in swap.c, but it exists
> even without CONFIG_SWAP, so that's OK.
> 
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> 
> diff -r abd2ad431e5c mm/swap.c
> --- a/mm/swap.c	Sat Apr 19 00:34:54 2008 +1000
> +++ b/mm/swap.c	Sat Apr 19 01:11:40 2008 +1000
> @@ -346,6 +346,7 @@ void release_pages(struct page **pages, 
>  
>  	pagevec_free(&pages_to_free);
>  }
> +EXPORT_SYMBOL(release_pages);
>  
>  /*
>   * The pages which we're about to release may be in the deferred lru-addition

acked-by: me.
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