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

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Date:   Sun, 25 Apr 2021 14:16:35 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     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

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.