linux-kernel - Re: [PATCH 08/17] {macvtap,tun}_get_user(): switch to iov

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Message-ID: <20141124101546.GE7996@ZenIV.linux.org.uk>
Date:	Mon, 24 Nov 2014 10:15:46 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	target-devel@...r.kernel.org, nab@...ux-iscsi.org,
	hch@...radead.org
Subject: Re: [PATCH 08/17] {macvtap,tun}_get_user(): switch to iov_iter

On Mon, Nov 24, 2014 at 12:27:42AM +0000, Ben Hutchings wrote:
> >  		copylen = vnet_hdr.hdr_len ? vnet_hdr.hdr_len : GOODCOPY_LEN;
> >  		if (copylen > good_linear)
> >  			copylen = good_linear;
> >  		linear = copylen;
> > -		if (iov_pages(iv, vnet_hdr_len + copylen, count)
> > -		    <= MAX_SKB_FRAGS)
> > +		i = *from;
> > +		iov_iter_advance(&i, copylen);
> > +		if (iov_iter_npages(&i, INT_MAX) <= MAX_SKB_FRAGS)
> 
> The maxpages argument should be MAX_SKB_FRAGS + 1 as we don't need the
> exact number.

In principle, that's true, but...  Do we really care?  It only buys you
anything if you have a monstrously fragmented iovec.  And if you end up
spending too considerable amount of time in that loop in iov_iter_npages,
you are by definition on the slow path - it *will* fail, since we do not
even try to merge adjacent iovec segments.  Never had...
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