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

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Date: Wed, 19 Jun 2024 18:25:32 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: Eric Dumazet <edumazet@...gle.com>
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 19/06/2024 17:51, Eric Dumazet 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.

Thanks Eric, that is indeed the missing piece.

I'll give it go.