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

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Date: Wed, 19 Jun 2024 16:51:56 +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
Subject: Re: [PATCH] net: micro-optimize skb_datagram_iter

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.