netdev - Re: [PATCH net] gve: Fix an edge case for TSO skb validity check

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Message-ID: <CA+f9V1NwSNpjMzCK2A3yjai4UoXPrq65=d1=wy50=o-EBvKoNQ@mail.gmail.com>
Date: Fri, 19 Jul 2024 07:31:32 -0700
From: Praveen Kaligineedi <pkaligineedi@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, willemb@...gle.com, shailend@...gle.com, 
	hramamurthy@...gle.com, csully@...gle.com, jfraker@...gle.com, 
	stable@...r.kernel.org, Bailey Forrest <bcf@...gle.com>, 
	Jeroen de Borst <jeroendb@...gle.com>
Subject: Re: [PATCH net] gve: Fix an edge case for TSO skb validity check

On Thu, Jul 18, 2024 at 8:47 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Thu, Jul 18, 2024 at 9:52 PM Praveen Kaligineedi
> <pkaligineedi@...gle.com> wrote:
> >
> > On Thu, Jul 18, 2024 at 4:07 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> >
> > > > +                      * segment, then it will count as two descriptors.
> > > > +                      */
> > > > +                     if (last_frag_size > GVE_TX_MAX_BUF_SIZE_DQO) {
> > > > +                             int last_frag_remain = last_frag_size %
> > > > +                                     GVE_TX_MAX_BUF_SIZE_DQO;
> > > > +
> > > > +                             /* If the last frag was evenly divisible by
> > > > +                              * GVE_TX_MAX_BUF_SIZE_DQO, then it will not be
> > > > +                              * split in the current segment.
> > >
> > > Is this true even if the segment did not start at the start of the frag?
> > The comment probably is a bit confusing here. The current segment
> > we are tracking could have a portion in the previous frag. The code
> > assumed that the portion on the previous frag (if present) mapped to only
> > one descriptor. However, that portion could have been split across two
> > descriptors due to the restriction that each descriptor cannot exceed 16KB.
>
> >>> /* If the last frag was evenly divisible by
> >>> +                                * GVE_TX_MAX_BUF_SIZE_DQO, then it will not be
> >>>  +                              * split in the current segment.
>
> This is true because the smallest multiple of 16KB is 32KB, and the
> largest gso_size at least for Ethernet will be 9K. But I don't think
> that that is what is used here as the basis for this statement?
>
The largest Ethernet gso_size (9K) is less than GVE_TX_MAX_BUF_SIZE_DQO
is an implicit assumption made in this patch and in that comment. Bailey,
please correct me if I am wrong..




> > That's the case this fix is trying to address.
> > I will work on simplifying the logic based on your suggestion below so
> > that the fix is easier to follow