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

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Date:   Mon, 1 Nov 2021 08:21:10 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     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 30.10.21 01:44, Boris Ostrovsky wrote:
> 
> 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.

BP_EAGAIN should not stop waiting, as it might be intermediate in case
some caches or buffers are freed.


Juergen

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