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

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Message-ID: <CAF=yD-JHDkDit0wPoKftTt3ZhtJ0gM3+E_YJsACKu916FpuCEg@mail.gmail.com>
Date: Sun, 21 Jul 2024 12:10:14 -0700
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Bailey Forrest <bcf@...gle.com>
Cc: Praveen Kaligineedi <pkaligineedi@...gle.com>, 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, 
	Jeroen de Borst <jeroendb@...gle.com>
Subject: Re: [PATCH net] gve: Fix an edge case for TSO skb validity check

On Fri, Jul 19, 2024 at 9:56 AM Bailey Forrest <bcf@...gle.com> wrote:
>
> On Fri, Jul 19, 2024 at 7:31 AM Praveen Kaligineedi
> <pkaligineedi@...gle.com> wrote:
> >
> > 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..
>
> If last_frag_size is evenly divisible by GVE_TX_MAX_BUF_SIZE_DQO, it
> doesn't hit the edge case we're looking for.
>
> - If it's evenly divisible, then we know it will use exactly
> (last_frag_size / GVE_TX_MAX_BUF_SIZE_DQO) descriptors

This assumes that gso_segment start is aligned with skb_frag
start. That is not necessarily true, right?

If headlen minus protocol headers is 1B, then the first segment
will have two descriptors { 1B, 9KB - 1 }. And the next segment
can have skb_frag_size - ( 9KB - 1).

I think the statement is correct, but because every multiple
of 16KB is so much larger than the max gso_size of ~9KB,
that a single segment will never include more than two
skb_frags.

Quite possibly the code overestimates the number of
descriptors per segment now, but that is safe and only a
performance regression.

> - GVE_TX_MAX_BUF_SIZE_DQO > 9k, so we know each descriptor won't
> create a segment which exceeds the limit

For a net patch, it is generally better to make a small fix rather than rewrite.

That said, my sketch without looping over every segment:

        while (off < skb->len) {
                gso_size_left = shinfo->gso_size;
                num_desc = 0;

                while (gso_size_left) {
                        desc_len = min(gso_size_left, frag_size_left);
                        gso_size_left -= desc_len;
                        frag_size_left -= desc_len;
                        num_desc++;

                        if (num_desc > max_descs_per_seg)
                                return false;

                        if (!frag_size_left)
                                frag_size_left =
skb_frag_size(&shinfo->frags[frag_idx++]);
+                      else
+                              frag_size_left %= gso_size;        /*
skip segments that fit in one desc */
                }
        }