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

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Date: Tue, 18 Jun 2024 09:37:12 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: Jakub Kicinski <kuba@...nel.org>, David Howells <dhowells@...hat.com>
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 17/06/2024 19:58, Jakub Kicinski 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.

dHowells?