netdev - Re: [PATCH] Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v2)

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Date:	Tue, 9 Nov 2010 16:20:00 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Maciej Żenczykowski <zenczykowski@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, eric.dumazet@...il.com
Subject: Re: [PATCH] Enhance AF_PACKET implementation to not require high
 order contiguous memory allocation (v2)

On Tue, Nov 09, 2010 at 01:07:32PM -0800, Maciej Żenczykowski wrote:
> > +#define PGV_FROM_VMALLOC 1
> 
> Why don't we always just use vmalloc, what's the benefit of get_user_pages?
> 
Because of how vmalloc works.  It maps discontiguous pages into contiguous
address space.  But we only have 128MB of that address space to work with by
default, so its quite possible that we won't be able to alloc all the memory.

> > +       /*
> > +        * vmalloc failed, lets dig into swap here
> > +        */
> > +       *flags = 0;
> 
> probably better to *flags &= ~PGV_FROM_VMALLOC;
> (since some flags could have been set before this function was called)
> 
Well, if any other users of this field existed, I'd agree, but since we're the
only one, I think its ok, at least for now.

> > +       gfp_flags &= ~__GFP_NORETRY;
> > +       buffer = (char *)__get_free_pages(gfp_flags, order);
> 
> wouldn't this still cause problems because you're now requiring linear
> memory again?
yes, its a last ditch effort after the other two options have been tried.  Its
all thats left to do.

> Would it be better to just fail at this point?
Why?  If we can dig into swap and get the memory, we may as well try.  It would
be better if we didn't have to, but if the choice is between failing and making
the system slow down....

Neil

> 
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