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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 09 March 2020 13:04
> To: paul@xxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Paul Durrant <pdurrant@xxxxxxxxxx>; 
> Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau Monné 
> <roger.pau@xxxxxxxxxx>
> Subject: Re: [PATCH v5 3/6] x86 / pv: do not treat PGC_extra pages as RAM
> 
> 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.

I thought the idea here was that the domheap dump loop should be dumping 
'normal' pages so it seems reasonable to me that the number of pages dumped to 
match the value returned by domain_tot_pages().
Would you perhaps be happier if we put 'extra' pages on separate page list, 
which can be unconditionally dumped so as I transition xenheap pages to 'extra' 
pages they don't get missed? It would also get rid of some of the other checks 
for PGC_extra that I have to introduce because they currently end up on the 
domain's page list.

  Paul

> 
> Jan


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