Re: [Xen-devel] balloon question

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 08.06.06 18:25 >>>
>
>On 8 Jun 2006, at 14:51, Jan Beulich wrote:
>
>> Running with Xen option dom0_mem=6G and kernel option mem=8G, I would 
>> have expected that I would see the system with a
>> 2G balloon right after boot. However, the balloon is empty. Briefly 
>> looking at this, it would seem to me that, with all
>> pages being reserved when the system starts and only the populated 
>> ones getting the reserved status cleared in
>> __free_pages_bootmem(), the condition to put pages into the balloon in 
>> balloon_init() is inverted.

>i386 deliberately marks pages beyond start_info->nr_pages as not 
>Reserved so they get picked up by the balloon driver. Either x86/64 
>needs to do the same, or we stop doing that in i386. But then there 
>will be pages that are legitimately Reserved that the balloon driver 

Where would, under Xen, such pages originate from? Namely beyond
start_info->nr_pages?

>will erroneously pick up. Or we could make the balloon driver 
>explicitly pick up all pages from start_info->nr_pages. Or have an 

It is already starting its scan at start_info->nr_pages.

>interface to allow arch code to register page ranges with the balloon 
>driver. This needs a little thought....
>
>> Also, dying in xen_destroy_contiguous_region() (and similarly in the 
>> failure recovery code in
>> xen_create_contiguous_region()) if the space just unmapped can't be 
>> re-populated seems overly harsh (we're having a case
>> where this actually happens); wouldn't it make sense to make this 
>> fatal only when the page can't be replaced by a free
>> one (allocated inside the domain, with the page stolen for this then 
>> simply pushed into the balloon, similar to
>> balloon_alloc_empty_page_range() except without freeing the memory to 
>> Xen)?
>
>As it happens I'm fixing this exact problem right now. :-) Expect a fix 
>in -unstable and -testing in the next few days.

Great!

Jan

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