linux-kernel - Re: [PATCH v7 01/31] iov_iter: Add ITER

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Date:   Sun, 25 Apr 2021 14:58:02 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        Dave Wysochanski <dwysocha@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 01/31] iov_iter: Add ITER_XARRAY

Al Viro <viro@...iv.linux.org.uk> wrote:

> > +struct address_space;
> >  struct pipe_inode_info;
> >  
> >  struct kvec {
> 
> What is that chunk for?

Ah, that can go.  It used to be ITER_MAPPING.

> > +		}),
> > +		({
> > +		rem = copy_mc_to_page(v.bv_page, v.bv_offset,
> > +				      (from += v.bv_len) - v.bv_len, v.bv_len);
> > +		if (rem) {
> > +			curr_addr = (unsigned long) from;
> > +			bytes = curr_addr - s_addr - rem;
> > +			rcu_read_unlock();
> > +			return bytes;
> > +		}
> 
> That's broken, same way as kvec and bvec cases are in the same primitive.
> Iterator not advanced on failure halfway through.

Okay.  I just copied what ITER_BVEC does.  Should this be handled in
iterate_and_advance() rather than in the code snippets it takes as parameters?

But for the moment, I guess I should just add:

	i->iov_offset += bytes;

to all three (kvec, bvec and xarray)?

> > @@ -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.

> > @@ -1268,7 +1372,9 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
> > ...
> Very limited use; it shouldn't be called for anything other than IOV_ITER case.
Should that just be cut down, then?

> > @@ -1849,7 +2111,12 @@ int iov_iter_for_each_range(struct iov_iter *i, size_t bytes,
> > ...
> 
> Would be easier to have that sucker removed first...

I could do that.  I'd rather not do that here since it hasn't sat in
linux-next, but since nothing uses it, but Linus might permit it.

David