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

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Date: Tue, 1 Aug 2023 12:16:23 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: 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

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.