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Date:   Tue, 10 Dec 2019 11:16:35 +0100
From:   Roger Pau =?iso-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>
To:     SeongJae Park <sj38.park@gmail.com>
CC:     <sjpark@amazon.com>, <axboe@kernel.dk>, <konrad.wilk@oracle.com>,
        <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <pdurrant@amazon.com>, <xen-devel@lists.xenproject.org>,
        SeongJae Park <sjpark@amazon.de>
Subject: Re: [PATCH v5 1/2] xenbus/backend: Add memory pressure handler
 callback
Message-ID: <20191210101635.GD980@Air-de-Roger>
References: <20191210080628.5264-1-sjpark@amazon.de>
 <20191210080628.5264-2-sjpark@amazon.de>
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On Tue, Dec 10, 2019 at 08:06:27AM +0000, SeongJae Park wrote:
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lack a

s/lack a/lack/

> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> a memory pressure and request specific devices of specific backend

s/monitor a/monitor/

> drivers which causing the given pressure to voluntarily release its

...which are causing...

> memory.
> 
> That said, this commit simply requests every callback registered driver
> to release its memory for every domain, rather than issueing the

s/issueing/issuing/

> requests to the drivers and the domain in charge.  Such things will be

I'm afraid I don't understand the "domain in charge" part of this
sentence.

> done in a futur.  Also, this commit focuses on memory only.  However, it

... done in a future change. Also I think the period after only should
be removed in order to tie both sentences together.

> would be ablt to be extended for general resources.

s/ablt/able/

> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>  drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
>  include/xen/xenbus.h                      |  1 +
>  2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..5a5ba29e39df 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>  	return NOTIFY_DONE;
>  }
>  
> +static int xenbus_backend_reclaim(struct device *dev, void *data)
> +{
> +	struct xenbus_driver *drv;

Newline and const.

> +	if (!dev->driver)
> +		return -ENOENT;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev));

You seem to completely ignore the return of the reclaim hook...

> +	return 0;
> +}
> +
> +/*
> + * Returns 0 always because we are using shrinker to only detect memory
> + * pressure.
> + */
> +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> +				struct shrink_control *sc)
> +{
> +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> +			xenbus_backend_reclaim);
> +	return 0;
> +}
> +
> +static struct shrinker xenbus_backend_shrinker = {
> +	.count_objects = xenbus_backend_shrink_count,
> +	.seeks = DEFAULT_SEEKS,
> +};
> +
>  static int __init xenbus_probe_backend_init(void)
>  {
>  	static struct notifier_block xenstore_notifier = {
> @@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
>  
>  	register_xenstore_notifier(&xenstore_notifier);
>  
> +	if (register_shrinker(&xenbus_backend_shrinker))
> +		pr_warn("shrinker registration failed\n");
> +
>  	return 0;
>  }
>  subsys_initcall(xenbus_probe_backend_init);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816d5f8c..cdb075e4182f 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -104,6 +104,7 @@ struct xenbus_driver {
>  	struct device_driver driver;
>  	int (*read_otherend_details)(struct xenbus_device *dev);
>  	int (*is_ready)(struct xenbus_device *dev);
> +	unsigned (*reclaim)(struct xenbus_device *dev);

... hence I wonder why it's returning an unsigned when it's just
ignored.

IMO it should return an int to signal errors, and the return should be
ignored.

Also, I think it would preferable for this function to take an extra
parameter to describe the resource the driver should attempt to free
(ie: memory or interrupts for example). I'm however not able to find
any existing Linux type to describe such resources.

Thanks, Roger.