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:

quoted

quoted

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?

quoted

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?