netdev - Re: [PATCH net v3] net: allow skb_datagram_iter to be called from any context

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Date: Wed, 26 Jun 2024 10:43:38 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Sagi Grimberg <sagi@...mberg.me>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>, 
	David Howells <dhowells@...hat.com>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH net v3] net: allow skb_datagram_iter to be called from any context

On Wed, Jun 26, 2024 at 10:23 AM Sagi Grimberg <sagi@...mberg.me> wrote:
>
>
>
> On 26/06/2024 10:40, Eric Dumazet wrote:
> > On Wed, Jun 26, 2024 at 9:00 AM Sagi Grimberg <sagi@...mberg.me> wrote:
> >> We only use the mapping in a single context, so kmap_local is sufficient
> >> and cheaper. Make sure to use skb_frag_foreach_page as skb frags may
> >> contain highmem compound pages and we need to map page by page.
> >>
> >> Reported-by: kernel test robot <oliver.sang@...el.com>
> >> Closes: https://lore.kernel.org/oe-lkp/202406161539.b5ff7b20-oliver.sang@intel.com
> >> Signed-off-by: Sagi Grimberg <sagi@...mberg.me>
> > Thanks for working on this !
> >
> > A patch targeting net tree would need a Fixes: tag, so that stable
> > teams do not have
> > to do archeological digging to find which trees need this fix.
>
> The BUG complaint was exposed by the reverted patch in net-next.
>
> TBH, its hard to tell when this actually was introduced, could skb_frags
> always
> have contained high-order pages? or was this introduced with the
> introduction
> of skb_copy_datagram_iter? or did it originate in the base implementation it
> was copied from skb_copy_datagram_const_iovec?

OK, I will therefore suggest something like this (even if the older
bug might be exposed
by a more recent patch), to cover all kernels after 5.0

This was the commit adding __skb_datagram_iter(), not the bug.

Fixes: 950fcaecd5cc ("datagram: consolidate datagram copy to iter helpers")

I suspect high-order highmem pages were not used in older kernels anyway.