Re: [Xen-devel] [PATCH v5 3/6] x86 / pv: do not treat PGC_extra pages as RAM

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.03.2020 11:23, paul@xxxxxxx wrote:
> From: Paul Durrant <pdurrant@xxxxxxxxxx>
> 
> This patch modifies several places walking the domain's page_list to make
> them ignore PGC_extra pages:
> 
> - dump_pageframe_info() should ignore PGC_extra pages in its dump as it
>   determines whether to dump using domain_tot_pages() which also ignores
>   PGC_extra pages.

This argument looks wrong to me: Let's take an example - a domain
almost fully cleaned up, with 8 "normal" and 3 "extra" pages left.
domain_tot_pages() returns 8 in this case, i.e. "normal" page
dumping doesn't get skipped. However, there now won't be any trace
of the "extra" pages, because they're also not on xenpage_list,
which gets all its pages dumped in all cases. Correct restoration
of original behavior would be to dump "normal" pages when there
are less than 10, and to dump all "extra" pages. (Same of course
goes for live domains, where "normal" page dumping would be
skipped in the common case, but xenheap pages would be dumped, and
hence so should be "extra" ones.) As indicated before, the removal
of the APIC assist page from xenpage_list was already slightly
regressing in this regard (as well as in at least one other way,
I'm afraid), and you're now deliberately making the regression
even bigger.

Jan

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