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Subject: Re: [PATCH v3] xen/balloon: add late_initcall_sync() for initial
 ballooning done
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org
References: <20211029142049.25198-1-jgross@suse.com>
 <11956c14-f1f7-70f0-40a6-aad31a264af6@oracle.com> <YXxzQhPvgAOkhGg/@mail-itl>
 <2365e65f-7431-4bf5-4ced-5e146776b9ac@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <0e32534b-c50f-d6ea-2927-dc763939dbcb@suse.com>
Date:   Mon, 1 Nov 2021 08:21:10 +0100
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From: Juergen Gross <jgross@suse.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= <marmarek@invisiblethingslab.com>
Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
 Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org
Message-ID: <0e32534b-c50f-d6ea-2927-dc763939dbcb@suse.com>
Subject: Re: [PATCH v3] xen/balloon: add late_initcall_sync() for initial
 ballooning done
References: <20211029142049.25198-1-jgross@suse.com>
 <11956c14-f1f7-70f0-40a6-aad31a264af6@oracle.com> <YXxzQhPvgAOkhGg/@mail-itl>
 <2365e65f-7431-4bf5-4ced-5e146776b9ac@oracle.com>
In-Reply-To: <2365e65f-7431-4bf5-4ced-5e146776b9ac@oracle.com>

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On 30.10.21 01:44, Boris Ostrovsky wrote:
>=20
> On 10/29/21 6:18 PM, Marek Marczykowski-G=C3=B3recki 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:
>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 Control=
 scrubbing pages before returning them to Xen for=20
>>>> others domains
>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 use. Ca=
n be set with xen_scrub_pages cmdline
>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 paramet=
er. Default value controlled with=20
>>>> CONFIG_XEN_SCRUB_PAGES_DEFAULT.
>>>> +
>>>> +What:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /sys/devices/system=
/xen_memory/xen_memory0/boot_timeout
>>>> +Date:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 November 2021
>>>> +KernelVersion:=C2=A0=C2=A0=C2=A0 5.16
>>>> +Contact:=C2=A0=C2=A0=C2=A0 xen-devel@lists.xenproject.org
>>>> +Description:
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 The time (in seconds) to=
 wait before giving up to boot in case
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 initial ballooning fails=
 to free enough memory. Applies only
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 when running as HVM or P=
VH guest and started with less memory
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 configured than allowed =
at max.
>>>
>>> How is this going to be used? We only need this during boot.
>>>
>>>
>>>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 state =3D update_schedul=
e(state);
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 balloon_state =3D update=
_schedule(balloon_state);
>>>
>>> Now that balloon_state has whole file scope it can probably be=20
>>> updated inside update_schedule().
>>>
>>>
>>>> +=C2=A0=C2=A0=C2=A0 while ((credit =3D current_credit()) < 0) {
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (credit !=3D last_cre=
dit) {
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =
last_changed =3D jiffies;
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =
last_credit =3D credit;
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (balloon_state =3D=3D=
 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:
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 balloo=
n_state =3D decrease_reservation(n_pages,
>>                                                             =20
>> GFP_BALLOON);
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (ba=
lloon_state =3D=3D BP_DONE && n_pages !=3D=20
>> -credit &&
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0 n_pages < totalreserve_pages)
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 balloon_state =3D BP_EAGAIN;
>>
>> It would be bad to finish waiting in this case.
>=20
>=20
> RIght, but if we were to say 'if (balloon_state !=3D BP_DONE)' the wors=
t=20
> that can happen is that we will continue on to the next iteration=20
> without warning and/or panicing. Of course, there is a chance thaton th=
e=20
> next iteration the same thing will happen but I think chances of hittin=
g=20
> this race every time are infinitely low. We can also check for=20
> current_credit() again.
>=20
>=20
> The question is whether we do want to continue waiting if we are in=20
> 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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