Re: [RFC PATCH 03/28] tcp: Support MSG_SPLICE_PAGES

David Howells wrote:

Willem de Bruijn [off-list ref] wrote:

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David Howells wrote:

quoted

Willem de Bruijn [off-list ref] wrote:

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The commit message mentions MSG_SPLICE_PAGES as an internal flag.

It can be passed from userspace. The code anticipates that and checks
preconditions.

Should I add a separate field in the in-kernel msghdr struct for such internal
flags?  That would also avoid putting an internal flag in the same space as
the uapi flags.

That would work, if no cost to common paths that don't need it.

Actually, it might be tricky.  __ip_append_data() doesn't take a msghdr struct
pointer per se.  The "void *from" argument *might* point to one - but it
depends on seeing a MSG_SPLICE_PAGES or MSG_ZEROCOPY flag, otherwise we don't
know.

Possibly this changes if sendpage goes away.

Is it sufficient to mask out this bit in tcp_sendmsg_locked and
udp_sendmsg if passed from userspace (and should be ignored), and pass
it through flags to callees like ip_append_data?

quoted

A not very pretty alternative would be to add an an extra arg to each
sendmsg handler that is used only when called from sendpage.

There are a few other internal MSG_.. flags, such as
MSG_SENDPAGE_NOPOLICY. Those are all limited to sendpage, and ignored
in sendmsg, I think. Which would explain why it was clearly safe to
add them.

Should those be moved across to the internal flags with MSG_SPLICE_PAGES?

I would not include that in this patch series.