Subject: Re: [RFC PATCH v1 2/2] virtio_net: Don't disable napi on low memory.

From: Rusty Russell <[email-masked]>
Date: Mon, 09 Jan 2012 17:16:52 +1030

On Fri, 6 Jan 2012 09:54:46 -0800, Mike Waychison <[email-masked]> wrote:
> On Wed, Jan 4, 2012 at 6:46 PM, Mike Waychison <[email-masked]> wrote:
> > On Wed, Jan 4, 2012 at 4:31 PM, Rusty Russell <[email-masked]> wrote:
> >> 4) You use the skb data for the linked list; use the skb head's list.
> 
> What did you mean by this?  I was under the impression that the ->next
> and ->prev fields in sk_buff were the first two elements specifically
> so that the pointer could be treated as a list_head.  If it's the cast
> in particular that you have an objection with, I can easily change
> this to a singly linked list threaded through ->next if that's
> cleaner.

Yep, I saw the cast and misread your code.  I could have sworn that skb
used a real list_head these days, but I'm wrong.

> >>
> >> Instead, here's how I think it should be done:
> ...
> >
> > This sounds reasonable to me.  I'll see what I can muster together this week.
> >
> 
> So I started implementing it the way you were mentioning, and ran into
> a problem with the original patchset.
> 
> Currently the "mergeable" and "big" receive buffers use a private page
> free list (virtnet_info->pages) which has no synchronization itself.
> This means that the batched version can't use get_a_page() and
> give_pages() as is, which reduces the need to re-use the same alloc
> halves that I've split.   Alternatives I can think of at this point:
> 
> - pass in a flag to the allocators like "bool is_serial" that is true
> if we are serializing with napi, (which determines if we can much with
> vi->pages)
> or
> - not use the same allocators for the "mergeable" and "big" paths.
> The mergeable allocator in the non-serialized case reduces to
> alloc_page(), while the big allocator looks like a copy and paste that
> uses alloc_page instead of get_a_page().
> 
> Preferences?  I'll code one of the two up and see what it looks like.

Whatever results in a cleaner driver, I'm happy.

Thanks,
Rusty.
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