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

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Date: Wed, 19 Jun 2024 16:54:22 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: David Howells <dhowells@...hat.com>, Jakub Kicinski <kuba@...nel.org>
Cc: 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



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?