Re: [Xen-devel] [PATCH v4 6/9] x86/mm: add an end_of_loop label in map_pages_to_xen

From: "Xia, Hongyan" <hongyxia@amazon.com>
To: "jbeulich@suse.com" <jbeulich@suse.com>,
	"Durrant, Paul" <pdurrant@amazon.com>
Cc: "andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"wl@xen.org" <wl@xen.org>,
	"roger.pau@citrix.com" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v4 6/9] x86/mm: add an end_of_loop label in map_pages_to_xen
Date: Thu, 5 Dec 2019 13:22:52 +0000	[thread overview]
Message-ID: <649e18f343bf9956c12046657bbd8cc4c5180396.camel@amazon.com> (raw)
In-Reply-To: <e09cf45c-1326-91b6-7602-5f0391dd22eb@suse.com>

I mean... I was taught so as well but I was also taught an exception
which is using it for error handling and cleaning up. I am not sure if
using alternatives would result in cleaner code in this situation.

Hongyan

On Thu, 2019-12-05 at 12:12 +0100, Jan Beulich wrote:
> On 05.12.2019 12:02, Durrant, Paul wrote:
> > > -----Original Message-----
> > > From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On
> > > Behalf Of Jan
> > > Beulich
> > > Sent: 05 December 2019 10:26
> > > To: Xia, Hongyan <hongyxia@amazon.com>
> > > Cc: andrew.cooper3@citrix.com; xen-devel@lists.xenproject.org; 
> > > wl@xen.org;
> > > roger.pau@citrix.com
> > > Subject: Re: [Xen-devel] [PATCH v4 6/9] x86/mm: add an
> > > end_of_loop label
> > > in map_pages_to_xen
> > > 
> > > On 05.12.2019 11:21, Xia, Hongyan wrote:
> > > > > On 02.10.2019 19:16, Hongyan Xia wrote:
> > > > > > We will soon need to clean up mappings whenever the out
> > > > > > most loop is
> > > > > > ended. Add a new label and turn relevant continue's into
> > > > > > goto's.
> > > > > 
> > > > > I think already when this still was RFC I did indicate that
> > > > > I'm not
> > > > > happy about the introduction of these labels (including also
> > > > > patch 8).
> > > > > I realize it's quite a lot to ask, but both functions would
> > > > > benefit
> > > > > from splitting up into per-level helper functions, which -
> > > > > afaict -
> > > > > would avoid the need for such labels, and which would at the
> > > > > same
> > > > > time likely make it quite a bit easier to extend these to the
> > > > > 5-level page tables case down the road.
> > > > 
> > > > A common pattern I have found when mapping PTE pages on-demand
> > > > (and I
> > > > think is the exact intention of these labels from Wei, also
> > > > described
> > > > in the commit message) is that we often need to do:
> > > > 
> > > > map some pages - process those pages - error occurs or this
> > > > iteration
> > > > of loop can be skipped - _clean up the mappings_ - continue or
> > > > return
> > > > 
> > > > As long as cleaning up is required, these labels will likely be
> > > > needed
> > > > as the clean-up path before skipping or returning, so I would
> > > > say we
> > > > will see such labels even if we split it into helper functions
> > > > (virt_to_xen_l[123]e() later in the patch series is an
> > > > example). I see
> > > > the labels more or less as orthogonal to modularising into
> > > > helper
> > > > functions.
> > > 
> > > I think differently: The fact that labels are needed is because
> > > of
> > > the complexity of the functions. Simpler functions would allow
> > > goto-free handling of such error conditions (by instead being
> > > able
> > > to use continue, break, or return without making the code less
> > > readable, often even improving readability).
> > 
> > And what is wrong with using goto-s? It is a *very* common style of
> > error handling use widely in e.g. the linux kernel. IMO it often
> > makes error paths much more obvious and easier to reason about. In
> > fact I very much dislike returns from the middle of functions as
> > they can easily lead to avoidance of necessary error cleanup.
> 
> Whereas I personally dislike goto-s (and I've been taught so when
> first learning programming languages). In private code I avoid them
> by all means. In projects I'm the maintainer for I accept them when
> the alternative is noticeably more ugly.
> 
> Jan
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