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

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Date: Thu, 27 Jun 2024 16:01:45 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Sagi Grimberg <sagi@...mberg.me>, netdev@...r.kernel.org,
	Jakub Kicinski <kuba@...nel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH net v3] net: allow skb_datagram_iter to be called from
 any context

On Wed, Jun 26, 2024 at 10:43:38AM +0200, Eric Dumazet wrote:
> 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.

This explanation is slightly garbled.

High-order lowmem pages _also_ get caught by this check.  See my earlier
email explaining this.