Re: [RFC] what to do with IOCB_DSYNC?

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, May 23, 2022 at 08:34:44AM -0600, Jens Axboe wrote:
> On 5/23/22 08:22, Al Viro wrote:
> > On Sun, May 22, 2022 at 08:43:26PM -0600, Jens Axboe wrote:
> > 
> >> Branch here:
> >>
> >> https://git.kernel.dk/cgit/linux-block/log/?h=iov-iter
> >>
> >> First 5 are generic ones, and some of them should just be folded with
> >> your changes.
> >>
> >> Last 2 are just converting io_uring to use it where appropriate.
> >>
> >> We can also use it for vectored readv/writev and recvmsg/sendmsg with
> >> one segment. The latter is mostly single segment in the real world
> >> anyway, former probably too. Though not sure it's worth it when we're
> >> copying a single iovec first anyway? Something to test...
> > 
> > Not a good idea.  Don't assume that all users of iov_iter are well-behaving;
> > not everything is flavour-agnostic.  If nothing else, you'll break the hell
> > out of infinibarf - both qib and hfi check that ->write_iter() gets
> > IOV_ITER target and fail otherwise.
> 
> OK, I'll check up on that.
> 
> > BTW, #work.iov_iter is going to be rebased and reordered; if nothing else,
> > a bunch of places like
> >         dio->should_dirty = iter_is_iovec(iter) && iov_iter_rw(iter) == READ;
> > need to be dealt with before we switch new_sync_read() and new_sync_write()
> > to ITER_UBUF.
> 
> I already made an attempt at that, see the git branch I sent in the last email.

There's several more, AFAICS (cifs, ceph, fuse, gfs2)...  The check in /dev/fuse
turned out to be fine - it's only using primitives, so we can pass ITER_UBUF
ones there.  mm/shmem.c check... similar, but I really wonder if x86 clear_user()
really sucks worse than copy_to_user() from zero page...

FWIW, I'd added bool ->user_backed next to ->data_source, with user_backed_iter()
as an inline predicate checking it.  Seems to get slightly better iov_iter.c
code generation that way...

Current branch pushed to #new.iov_iter (at the moment; will rename back to work.iov_iter
once it gets more or less stable).