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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan

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