netdev - Re: [PATCH v2] tun: avoid high-order page allocation for packet header

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Date: Tue, 01 Aug 2023 09:14:08 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 Tahsin Erdogan <trdgn@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Herbert Xu <herbert@...dor.apana.org.au>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tun: avoid high-order page allocation for packet
 header

Eric Dumazet wrote:
> On Tue, Aug 1, 2023 at 11:37 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Aug 1, 2023 at 1:07 AM Tahsin Erdogan <trdgn@...zon.com> wrote:
> > >
> > > When GSO is not enabled and a packet is transmitted via writev(), all
> > > payload is treated as header which requires a contiguous memory allocation.
> > > This allocation request is harder to satisfy, and may even fail if there is
> > > enough fragmentation.
> > >
> > > Note that sendmsg() code path limits the linear copy length, so this change
> > > makes writev() and sendmsg() more consistent.
> > >
> > > Signed-off-by: Tahsin Erdogan <trdgn@...zon.com>
> > > ---
> >
> > I will have to tweak one existing packetdrill test, nothing major.
> >
> > Tested-by: Eric Dumazet <edumazet@...gle.com>
> > Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> I have to take this back, sorry.
> 
> We need to change alloc_skb_with_frags() and tun.c to attempt
> high-order allocations,
> otherwise tun users sending very large buffers will regress.
> (Even if this _could_ fail as you pointed out if memory is tight/fragmented)
> 
> I am working to make the change in alloc_skb_with_frags() and in tun,
> we can apply your patch after this prereq.

This exactly same allocation logic also exists in packet_alloc_skb and
tap_alloc_skb. If changing one of them, perhaps should address convert
all at the same time, to keep behavior consistent.