Re: [PATCH v7 01/31] iov_iter: Add ITER_XARRAY

Al Viro
2021-04-25T14:16:43Z

On Sun, Apr 25, 2021 at 02:58:02PM +0100, David Howells wrote:

> But for the moment, I guess I should just add:
>
> i->iov_offset += bytes;
>
> to all three (kvec, bvec and xarray)?

No. First of all, you'd need ->count updated as well; for kvec and bvec you
*REALLY* don't have to end up with ->iov_offset exceeding the size of current
kvec or bvec resp.; Bad Shit(tm) happens that way.

>
> > > @@ -1246,7 +1349,8 @@ unsigned long iov_iter_alignment(const struct iov_iter *i)
> > > iterate_all_kinds(i, size, v,
> > > (res |= (unsigned long)v.iov_base | v.iov_len, 0),
> > > res |= v.bv_offset | v.bv_len,
> > > - res |= (unsigned long)v.iov_base | v.iov_len
> > > + res |= (unsigned long)v.iov_base | v.iov_len,
> > > + res |= v.bv_offset | v.bv_len
> > > )
> > > return res;
> > > }
> >
> > Hmm... That looks like a really bad overkill - do you need anything beyond
> > count and iov_offset in that case + perhaps "do we have the very last page"?
> > IOW, do you need to iterate anything at all here? What am I missing here?
>
> Good point. I wonder, even, if the alignment could just be set to 1. There's
> no kdoc description on the function that says what the result is meant to
> represent.

Huh? It's the worst alignment of all segment boundaries, what else? As in
if (iov_iter_alignment(i) & 1023)
// we have something in there that isn't 1K-aligned.