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

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Date: Wed, 19 Jun 2024 13:46:17 +0100
From: David Howells <dhowells@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: dhowells@...hat.com, Sagi Grimberg <sagi@...mberg.me>,
    Matthew Wilcox <willy@...radead.org>,
    kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
    lkp@...el.com, netdev@...r.kernel.org,
    Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] net: micro-optimize skb_datagram_iter

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.

David