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SubjectRe: [PATCH v3] xen/balloon: add late_initcall_sync() for initial ballooning done
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On 10/29/21 6:18 PM, Marek Marczykowski-Górecki wrote:
> On Fri, Oct 29, 2021 at 05:46:18PM -0400, Boris Ostrovsky wrote:
>> On 10/29/21 10:20 AM, Juergen Gross wrote:
>>> --- a/Documentation/ABI/stable/sysfs-devices-system-xen_memory
>>> +++ b/Documentation/ABI/stable/sysfs-devices-system-xen_memory
>>> @@ -84,3 +84,13 @@ Description:
>>> Control scrubbing pages before returning them to Xen for others domains
>>> use. Can be set with xen_scrub_pages cmdline
>>> parameter. Default value controlled with CONFIG_XEN_SCRUB_PAGES_DEFAULT.
>>> +
>>> +What: /sys/devices/system/xen_memory/xen_memory0/boot_timeout
>>> +Date: November 2021
>>> +KernelVersion: 5.16
>>> +Contact: xen-devel@lists.xenproject.org
>>> +Description:
>>> + The time (in seconds) to wait before giving up to boot in case
>>> + initial ballooning fails to free enough memory. Applies only
>>> + when running as HVM or PVH guest and started with less memory
>>> + configured than allowed at max.
>>
>> How is this going to be used? We only need this during boot.
>>
>>
>>> - state = update_schedule(state);
>>> + balloon_state = update_schedule(balloon_state);
>>
>> Now that balloon_state has whole file scope it can probably be updated inside update_schedule().
>>
>>
>>> + while ((credit = current_credit()) < 0) {
>>> + if (credit != last_credit) {
>>> + last_changed = jiffies;
>>> + last_credit = credit;
>>> + }
>>> + if (balloon_state == BP_ECANCELED) {
>>
>> What about other states? We are really waiting for BP_DONE, aren't we?
> BP_DONE is set also as an intermediate step:
>
> balloon_state = decrease_reservation(n_pages,
> GFP_BALLOON);
> if (balloon_state == BP_DONE && n_pages != -credit &&
> n_pages < totalreserve_pages)
> balloon_state = BP_EAGAIN;
>
> It would be bad to finish waiting in this case.


RIght, but if we were to say 'if (balloon_state != BP_DONE)' the worst that can happen is that we will continue on to the next iteration without warning and/or panicing. Of course, there is a chance thaton the next iteration the same thing will happen but I think chances of hitting this race every time are infinitely low. We can also check for current_credit() again.


The question is whether we do want to continue waiting if we are in BP_AGAIN. I don't think BP_WAIT is possible in this case although this may change in the future and we will forget to update this code.


-boris

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