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Subject: Re: [PATCH 2/4] xen/blkfront: cleanup stale persistent grants
To:     =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-block@vger.kernel.org, konrad.wilk@oracle.com,
        axboe@kernel.dk, boris.ostrovsky@oracle.com
References: <20180806113403.24728-1-jgross@suse.com>
 <20180806113403.24728-4-jgross@suse.com>
 <20180806161638.nmjamflckekeuyzb@mac>
From:   Juergen Gross <jgross@suse.com>
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Message-ID: <e28d3e77-2c6b-fc30-37e6-646ee1cfbdab@suse.com>
Date:   Tue, 7 Aug 2018 08:31:31 +0200
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On 06/08/18 18:16, Roger Pau Monné wrote:
> On Mon, Aug 06, 2018 at 01:34:01PM +0200, Juergen Gross wrote:
>> Add a periodic cleanup function to remove old persistent grants which
>> are no longer in use on the backend side. This avoids starvation in
>> case there are lots of persistent grants for a device which no longer
>> is involved in I/O business.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>  drivers/block/xen-blkfront.c | 99 ++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 95 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index b5cedccb5d7d..19feb8835fc4 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -46,6 +46,7 @@
>>  #include <linux/scatterlist.h>
>>  #include <linux/bitmap.h>
>>  #include <linux/list.h>
>> +#include <linux/workqueue.h>
>>  
>>  #include <xen/xen.h>
>>  #include <xen/xenbus.h>
>> @@ -121,6 +122,9 @@ static inline struct blkif_req *blkif_req(struct request *rq)
>>  
>>  static DEFINE_MUTEX(blkfront_mutex);
>>  static const struct block_device_operations xlvbd_block_fops;
>> +static struct delayed_work blkfront_work;
>> +static LIST_HEAD(info_list);
>> +static bool blkfront_work_active;
>>  
>>  /*
>>   * Maximum number of segments in indirect requests, the actual value used by
>> @@ -216,6 +220,7 @@ struct blkfront_info
>>  	/* Save uncomplete reqs and bios for migration. */
>>  	struct list_head requests;
>>  	struct bio_list bio_list;
>> +	struct list_head info_list;
>>  };
>>  
>>  static unsigned int nr_minors;
>> @@ -1764,6 +1769,12 @@ static int write_per_ring_nodes(struct xenbus_transaction xbt,
>>  	return err;
>>  }
>>  
>> +static void free_info(struct blkfront_info *info)
>> +{
>> +	list_del(&info->info_list);
>> +	kfree(info);
>> +}
>> +
>>  /* Common code used when first setting up, and when resuming. */
>>  static int talk_to_blkback(struct xenbus_device *dev,
>>  			   struct blkfront_info *info)
>> @@ -1885,7 +1896,10 @@ static int talk_to_blkback(struct xenbus_device *dev,
>>   destroy_blkring:
>>  	blkif_free(info, 0);
>>  
>> -	kfree(info);
>> +	mutex_lock(&blkfront_mutex);
>> +	free_info(info);
>> +	mutex_unlock(&blkfront_mutex);
>> +
>>  	dev_set_drvdata(&dev->dev, NULL);
>>  
>>  	return err;
>> @@ -1996,6 +2010,10 @@ static int blkfront_probe(struct xenbus_device *dev,
>>  	info->handle = simple_strtoul(strrchr(dev->nodename, '/')+1, NULL, 0);
>>  	dev_set_drvdata(&dev->dev, info);
>>  
>> +	mutex_lock(&blkfront_mutex);
>> +	list_add(&info->info_list, &info_list);
>> +	mutex_unlock(&blkfront_mutex);
>> +
>>  	return 0;
>>  }
>>  
>> @@ -2306,6 +2324,15 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
>>  	if (indirect_segments <= BLKIF_MAX_SEGMENTS_PER_REQUEST)
>>  		indirect_segments = 0;
>>  	info->max_indirect_segments = indirect_segments;
>> +
>> +	if (info->feature_persistent) {
>> +		mutex_lock(&blkfront_mutex);
>> +		if (!blkfront_work_active) {
>> +			blkfront_work_active = true;
>> +			schedule_delayed_work(&blkfront_work, HZ * 10);
> 
> Does it make sense to provide a module parameter to rune the schedule
> of the cleanup routine?

I don't think this is something anyone would like to tune.

In case you think it should be tunable I can add a parameter, of course.

> 
>> +		}
>> +		mutex_unlock(&blkfront_mutex);
> 
> Is it really necessary to have the blkfront_work_active boolean? What
> happens if you queue the same delayed work more than once?

In case there is already work queued later calls of
schedule_delayed_work() will be ignored.

So yes, I can drop the global boolean (I still need a local flag in
blkfront_delay_work() for controlling the need to call
schedule_delayed_work() again).


Juergen