linux-kernel - Re: [PATCH v3] xen/balloon: add late_initcall

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Date:   Fri, 29 Oct 2021 19:44:31 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] xen/balloon: add late_initcall_sync() for initial
 ballooning done


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@...ts.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