Messages in this thread Patch in this message | | | Date | Mon, 13 Jun 2022 07:42:45 -0700 | From | Joe Damato <> | Subject | Re: [RFC,iov_iter v2 3/8] iov_iter: add copyin_iovec helper |
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On Mon, Jun 13, 2022 at 07:53:19AM +0000, David Laight wrote: > From: Joe Damato > > Sent: 12 June 2022 09:58 > > > > copyin_iovec is a helper which wraps copyin and selects the right copy > > method based on the iter_copy_type. > > A pretty bad description.
Thanks, David. I'll be sure to fix the commit description in the next revision.
> > Signed-off-by: Joe Damato <jdamato@fastly.com> > > --- > > lib/iov_iter.c | 19 ++++++++++++++----- > > 1 file changed, 14 insertions(+), 5 deletions(-) > > > > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > > index d32d7e5..6720cb2 100644 > > --- a/lib/iov_iter.c > > +++ b/lib/iov_iter.c > > @@ -168,6 +168,15 @@ static int copyin(void *to, const void __user *from, size_t n) > > return n; > > } > > > > +static int copyin_iovec(void *to, const void __user *from, size_t n, > > + struct iov_iter *i) > > +{ > > + if (unlikely(iov_iter_copy_is_nt(i))) > > + return __copy_from_user_nocache(to, from, n); > > + else > > + return copyin(to, from, n); > > +} > > Isn't this extra conditional going to have a measurable impact > on all the normal copy paths?
The kernel already does a conditional for tx-nocache-copy on TCP sockets when copying skbs to check for the NETIF_F_NOCACHE_COPY bit, but I hear what you are saying.
I suppose I could push the NT copy check logic out of iov_iter, but to do that I think I'd probably have to significantly refactor the iov code to break apart copy_page_from_iter_iovec.
I'll spend a bit more time thinking through this, but I'm open to suggestions if you have one; the benefit of supporting non-temporal copies in this path is pretty significant, so I hope a path forward can be found.
> The additional costs of all the 'iovec' types is bad enough > already.
Do you have data you can share on this?
Thanks for taking a look!
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