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Date: Mon, 17 Jun 2024 09:58:52 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Matthew Wilcox <willy@infradead.org>, kernel test robot
 <oliver.sang@intel.com>, oe-lkp@lists.linux.dev, lkp@intel.com,
 netdev@vger.kernel.org, Eric Dumazet <edumazet@google.com>, David Howells
 <dhowells@redhat.com>
Subject: Re: [PATCH] net: micro-optimize skb_datagram_iter
Message-ID: <20240617095852.66c96be9@kernel.org>
In-Reply-To: <033294ee-e6e6-4dca-b60c-019cb72a6857@grimberg.me>
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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.