On Wed, 2014-01-08 at 10:28 -0800, Michael Dalton wrote:

Hi Jason,

On Tue, Jan 7, 2014 at 10:23 PM, Jason Wang [off-list ref] wrote:

quoted

What's the reason that this extra space is not accounted for truesize?

The initial rationale was that this extra space is due to
internal fragmentation in the page frag allocator, but I agree with
you -- this code should be changed and the extra space accounted for.
Any internal fragmentation leading to a larger last packet allocated from
the page should be reflected in the SKB truesize of the last packet.

Thats not a hard requirement.

Lets keep things simple and fast.

Correctness here is not mandatory, as long as we do not underestimate
truesize by a huge value like a factor of 10.

Hint :

When __netdev_alloc_frag() has to allocate a new page, because
remaining space in current page is too small, the lost hole is not
accounted to users of current page.