2011/12/22 David Miller [off-list ref]:

From: Michał Mirosław <redacted>
Date: Thu, 22 Dec 2011 20:29:30 +0100

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2011/12/22 David Miller [off-list ref]:

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From: Michał Mirosław <redacted>
Date: Thu, 22 Dec 2011 19:34:21 +0100

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2011/12/22 Ian Campbell [off-list ref]:

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On Wed, 2011-12-21 at 19:28 +0000, David Miller wrote:

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From: Eric Dumazet <redacted>
Date: Wed, 21 Dec 2011 15:02:18 +0100

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No idea on this +2 point.

I think I know, and I believe I instructed Alexey Kuznetsov to do
this.

When sendfile() is performed, we might start the SKB with the last few
bytes of one page, and end the SKB with the first few bytes of another
page.

In order to fit a full 64K frame into an SKB in this situation we have
to accomodate this case.

Thanks David, that makes sense.

However I think you only actually need 1 extra page for that. If the
data in frag[0] starts at $offset then frag[16] will need to have
$offset bytes in it. e.g.
       4096-$offset + 4096*15 + $offset = 65536
which == 17 pages rather than 18.

The following documents the status quo but I could update to switch to +
1 instead if there are no flaws in the above logic...

Since max IP datagram is 64K-1, adding ethernet and possibly VLAN
headers makes the max size slightly above 64K and then you have
64K/PAGE_SIZE+2 pages appear in worst case.

Headers go into the linear area, so they are not relevant for these
calculations.

Does this hold for LRO'ed packets and packets sent via PF_PACKET socket?

LRO is for receive, not packets we build on transmit, and in any event
have their headers also pulled into the linear area before the stack
sees it.

Yes, but the drivers seem to first fill the data to frags[] and only
then move the packets header (I browsed myri10ge).

For PF_PACKET, in the packet_snd() case it uses a linear SKB and in
the tpacket_snd() case it uses a linear SKB as well.

I saw skb_fill_page_desc() in af_packet.c (called by
tpacket_fill_skb() which is called by tpacket_snd()), that's why I'm
asking.

Best Regards,
Michał Mirosław