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>
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 10:21:56 +0000	[thread overview]
Message-ID: <f13c0e4808e320a0233f82b9be005fe5b2679469.camel@amazon.com> (raw)
In-Reply-To: <3885863bfc54a5f5f05cddb3cd9afe24897f27b3.1575477921.git.hongyxia@amazon.com>

>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.
>
>Thoughts?
>
>Jan

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 would also like to see some helpers because these functions are
getting too long and convoluted, but I wonder if they could be another
mini-series outside the directmap-removal series.

Hongyan
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