linux-kernel - Re: [Patch V2 2/2] xen: before ballooning hotplugged memory, set frames to invalid

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Date:	Fri, 20 Mar 2015 15:35:46 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com,
	daniel.kiper@...cle.com, pebolle@...cali.nl
Subject: Re: [Patch V2 2/2] xen: before ballooning hotplugged memory, set
 frames to invalid

On 03/20/2015 02:44 PM, Boris Ostrovsky wrote:
> On 03/20/2015 08:55 AM, Juergen Gross wrote:
>> Commit 25b884a83d487fd62c3de7ac1ab5549979188482 ("x86/xen: set
>> regions above the end of RAM as 1:1") introduced a regression.
>>
>> To be able to add memory pages which were added via memory hotplug to
>> a pv domain, the pages must be "invalid" instead of "identity" in the
>> p2m list before they can be added.
>>
>> Suggested-by: David Vrabel <david.vrabel@...rix.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>   drivers/xen/balloon.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index 0b52d92..65fedb8 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -229,6 +229,19 @@ static enum bp_state
>> reserve_additional_memory(long credit)
>>       balloon_hotplug = round_up(balloon_hotplug, PAGES_PER_SECTION);
>>       nid = memory_add_physaddr_to_nid(hotplug_start_paddr);
>> +#ifdef CONFIG_XEN_HAVE_PVMMU
>> +    if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>> +        unsigned long pfn, i;
>> +
>> +        pfn = PFN_DOWN(hotplug_start_paddr);
>> +        for (i = 0; i < balloon_hotplug; i++)
>> +            if (!set_phys_to_machine(pfn + i, INVALID_P2M_ENTRY)) {
>> +                pr_warn("set_phys_to_machine() failed, no memory
>> added\n");
>> +                return BP_ECANCELED;
>
> I should have asked last time --- should we restore [0..i-1] mapping
> back to identity on error here? I wonder whether leaving those frames as
> invalid (when they are expected to be 'identity') could cause problems
> in the future.

The only reason why set_phys_to_machine() can fail is a lack of memory
(which in this case would really be funny: adding memory fails due to a
lack of memory). In case a retry succeeds everything is fine as setting
an invalid entry to invalid again does no harm. In case the invalid
entry does any harm it would have done so without set_phys_to_machine()
failing. So I see no reason to restore the identity entries.

Juergen

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