netdev - Re: [PATCH] net: micro-optimize skb_datagram

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Date: Wed, 19 Jun 2024 16:56:15 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Sagi Grimberg <sagi@...mberg.me>
Cc: David Howells <dhowells@...hat.com>, Jakub Kicinski <kuba@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>, kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev, 
	lkp@...el.com, netdev@...r.kernel.org, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH] net: micro-optimize skb_datagram_iter

On Wed, Jun 19, 2024 at 4:51 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jun 19, 2024 at 3:54 PM Sagi Grimberg <sagi@...mberg.me> wrote:
> >
> >
> >
> > On 19/06/2024 15:46, David Howells wrote:
> > > Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > >> On Mon, 17 Jun 2024 09:29:53 +0300 Sagi Grimberg wrote:
> > >>>> Probably because kmap() returns page_address() for non-highmem pages
> > >>>> while kmap_local_page() actually returns a kmap address:
> > >>>>
> > >>>>           if (!IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) && !PageHighMem(page))
> > >>>>                   return page_address(page);
> > >>>>           return __kmap_local_pfn_prot(page_to_pfn(page), prot);
> > >>>>
> > >>>> so if skb frags are always lowmem (are they?) this is a false positive.
> > >>> AFAIR these buffers are coming from the RX ring, so they should be
> > >>> coming from a page_frag_cache,
> > >>> so I want to say always low memory?
> > >>>
> > >>>> if they can be highmem, then you've uncovered a bug that nobody's
> > >>>> noticed because nobody's testing on 32-bit any more.
> > >>> Not sure, Jakub? Eric?
> > >> My uneducated guess would be that until recent(ish) sendpage rework
> > >> from David Howells all high mem pages would have been single pages.
> > > Um.  I touched the Tx side, not the Rx side.
> > >
> > > I also don't know whether all high mem pages would be single pages.  I'll have
> > > to defer that one to the MM folks.
> >
> > What prevents from gro to expand frags from crossing PAGE_SIZE?
> >
> > btw, at least from the code in skb_gro_receive() it appears that
> > page_address() is called directly,
> > which suggest that these netmem pages are lowmem?
>
> GRO should only be fed with lowmem pages.
>
> But the trace involves af_unix, not GRO ?
>
> I guess that with splice games, it is possible to add high order pages to skbs.
>
> I think skb_frag_foreach_page() could be used to fix this issue.

For reference, please look at

commit c613c209c3f351d47158f728271d0c73b6dd24c6
Author: Willem de Bruijn <willemb@...gle.com>
Date:   Mon Jul 31 08:15:47 2017 -0400

    net: add skb_frag_foreach_page and use with kmap_atomic