RE: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw()

David Laight
2020-09-21T15:44:09Z

From: Al Viro
> Sent: 21 September 2020 16:30
>
> On Mon, Sep 21, 2020 at 03:21:35PM +0000, David Laight wrote:
>
> > You really don't want to be looping through the array twice.
>
> Profiles, please.

I did some profiling of send() v sendmsg() much earlier in the year.
I can't remember the exact details but the extra cost of sendmsg()
is far more than you might expect.
(I was timing sending fully built IPv4 UDP packets using a raw socket.)

About half the difference does away if you change the
copy_from_user() to __copy_from_user() when reading the struct msghdr
and iov[] from userspace (user copy hardening is expensive).

The rest is just code path, my gut feeling is that a lot of that
is in import_iovec().

Remember semdmsg() is likely to be called with an iov count of 1.

David

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