Re: problems with e1000 and jumboframes

From: Evgeniy Polyakov
Date: Thu Aug 03 2006 - 11:01:32 EST

On Thu, Aug 03, 2006 at 04:37:35PM +0200, Arnd Hannemann (arnd@xxxxxxxxxx) wrote:
> >> im running vanilla and if i try to set the mtu of my e1000 nic
> >> to 9000 bytes, page allocation failures occur (see below).
> >>
> >> However the box is a VIA Epia MII12000 with 1 GB of Ram and 1 GB of swap
> >> enabled, so there should be plenty of memory available. HIGHMEM support
> >> is off. The e1000 nic seems to be an 82540EM, which to my knowledge
> >> should support jumboframes.
> >
> > But it does not support splitting them into page sized chunks, so it
> > requires the whole jumbo frame allocation in one contiguous chunk, 9k
> > will be transferred into 16k allocation (order 3), since SLAB uses
> > power-of-2 allocation.
> Hmm, ok, what is the meaning of this line then:
> > Normal: 44578*4kB 11117*8kB 800*16kB 0*32kB 1*64kB 1*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 280240kB
> Are this the allocations which already happend? I thought they would
> represent the free memory, not the already used one?

3-order is 32k actually.

> >> However I can't always reproduce this on a freshly booted system, so
> >> someone else may be the culprit and leaking pages?
> >
> > You will almost 100% reproduce it after "find / > /dev/null".
> >
> >> Any ideas how to debug this?
> >
> > It can not be debugged - you have cought a memory fragmentation problem,
> > which is quite common.
> That's too bad :-(
> However it seems hard for me to imagine why there is no contiguous chunk
> of 16k when there are hundreds of Mbyte free. Can't those other pages be
> moved by the kernel, if a higher order allocation is requested?

e1000 is trying to allocate 32k, not 16 for jumbo frames.

> >>> kswapd0: page allocation failure. order:3, mode:0x20
> >
> > e1000 tries to allocate 3-order pages atomically?
> > Well, that's wrong.
> >
> Why? After your explanation that makes sense for me. The driver needs
> one contiguous chunk for those 9k packet buffer and thus requests a
> 3-order page of 16k. Or do i still do not understand this?

Correct, except that it wants 32k.
e1000 logic is following:
align frame size to power-of-two, then skb_alloc adds a little
(sizeof(struct skb_shared_info)) at the end, and this ends up
in 32k request just for 9k jumbo frame.

And it wants it in atomic context.

Evgeniy Polyakov
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