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Subject: Re: [PATCH v4 2/7] Enable balloon drivers to report inflated memory
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20221005090158.2801592-3-alexander.atanasov@virtuozzo.com>
Date:   Wed, 5 Oct 2022 10:25:47 -0700
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, kernel@openvz.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
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Message-Id: <88EDC41D-408F-4ADF-A933-0A6F36E5F262@gmail.com>
References: <20221005090158.2801592-1-alexander.atanasov@virtuozzo.com>
 <20221005090158.2801592-3-alexander.atanasov@virtuozzo.com>
To:     Alexander Atanasov <alexander.atanasov@virtuozzo.com>
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On Oct 5, 2022, at 2:01 AM, Alexander Atanasov =
<alexander.atanasov@virtuozzo.com> wrote:

> Add counters to be updated by the balloon drivers.
> Create balloon notifier to propagate changes.

I missed the other patches before (including this one). Sorry, but next
time, please cc me.

I was looking through the series and I did not see actual users of the
notifier. Usually, it is not great to build an API without users.

[snip]=20

> +
> +static int balloon_notify(unsigned long val)
> +{
> +	return srcu_notifier_call_chain(&balloon_chain, val, NULL);

Since you know the inflated_kb value here, why not to use it as an =
argument
to the callback? I think casting to (void *) and back is best. But you =
can
also provide pointer to the value. Doesn=E2=80=99t it sound better than =
having
potentially different notifiers reading different values?

Anyhow, without users (actual notifiers) it=E2=80=99s kind of hard to =
know how
reasonable it all is. For instance, is it balloon_notify() supposed to
prevent further balloon inflating/deflating until the notifier =
completes?
Accordingly, are callers to balloon_notify() expected to relinquish =
locks
before calling balloon_notify() to prevent deadlocks and high latency?