Re: [RFC] what to do with IOCB_DSYNC?

[Matthew Wilcox <willy@infradead.org>]

On Sun, May 22, 2022 at 10:36:00AM +0000, Al Viro wrote:
> On Sun, May 22, 2022 at 11:23:43AM +0100, Matthew Wilcox wrote:
> > On Sun, May 22, 2022 at 09:45:09AM +0200, Christoph Hellwig wrote:
> > > On Sat, May 21, 2022 at 04:14:07PM -0600, Jens Axboe wrote:
> > > > Then we're almost on par, and it looks like we just need to special case
> > > > iov_iter_advance() for the nr_segs == 1 as well to be on par. This is on
> > > > top of your patch as well, fwiw.
> > > > 
> > > > It might make sense to special case the single segment cases, for both
> > > > setup, iteration, and advancing. With that, I think we'll be where we
> > > > want to be, and there will be no discernable difference between the iter
> > > > paths and the old style paths.
> > > 
> > > A while ago willy posted patches to support a new ITER type for direct
> > > userspace pointer without iov.  It might be worth looking through the
> > > archives and test that.
> > 
> > https://lore.kernel.org/linux-fsdevel/Yba+YSF6mkM%2FGYlK@casper.infradead.org/
> 
> 	Direct kernel pointer, surely?  And from a quick look,
> iov_iter_is_kaddr() checks for the wrong value...

Indeed.  I didn't test it; it was a quick patch to see if the idea was
worth pursuing.  Neither you nor Christoph thought so at the time, so
I dropped it.  if there are performance improvements to be had from
doing something like that, it's a more compelling idea than just "Hey,
this removes a few lines of code and a bit of stack space from every
caller".