linux-kernel - Re: [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user requested

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Message-ID: <20220831155045.kxopdchlc67fmi5n@yadavpratyush.com>
Date:   Wed, 31 Aug 2022 15:50:45 +0000
From:   Pratyush Yadav <ptyadav@...zon.de>
To:     SeongJae Park <sj@...nel.org>
CC:     <jgross@...e.com>, <roger.pau@...rix.com>,
        <marmarek@...isiblethingslab.com>, <mheyne@...zon.de>,
        <xen-devel@...ts.xenproject.org>, <axboe@...nel.dk>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user
 requested

On 25/08/22 04:15PM, SeongJae Park wrote:
> Commit e94c6101e151 ("xen-blkback: Apply 'feature_persistent' parameter
> when connect") made blkback to advertise its support of the persistent
> grants feature only if the user sets the 'feature_persistent' parameter
> of the driver and the frontend advertised its support of the feature.
> However, following commit 402c43ea6b34 ("xen-blkfront: Apply
> 'feature_persistent' parameter when connect") made the blkfront to work
> in the same way.  That is, blkfront also advertises its support of the
> persistent grants feature only if the user sets the 'feature_persistent'
> parameter of the driver and the backend advertised its support of the
> feature.
> 
> Hence blkback and blkfront will never advertise their support of the
> feature but wait until the other advertises the support, even though
> users set the 'feature_persistent' parameters of the drivers.  As a
> result, the persistent grants feature is disabled always regardless of
> the 'feature_persistent' values[1].
> 
> The problem comes from the misuse of the semantic of the advertisement
> of the feature.  The advertisement of the feature should means only
> availability of the feature not the decision for using the feature.
> However, current behavior is working in the wrong way.
> 
> This commit fixes the issue by making the blkfront advertises its
> support of the feature as user requested via 'feature_persistent'
> parameter regardless of the otherend's support of the feature.
> 
> [1] https://lore.kernel.org/xen-devel/bd818aba-4857-bc07-dc8a-e9b2f8c5f7cd@suse.com/
> 
> Fixes: 402c43ea6b34 ("xen-blkfront: Apply 'feature_persistent' parameter when connect")
> Cc: <stable@...r.kernel.org> # 5.10.x
> Reported-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
> Suggested-by: Juergen Gross <jgross@...e.com>
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  drivers/block/xen-blkfront.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 8e56e69fb4c4..dfae08115450 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -213,6 +213,9 @@ struct blkfront_info
>  	unsigned int feature_fua:1;
>  	unsigned int feature_discard:1;
>  	unsigned int feature_secdiscard:1;
> +	/* Connect-time cached feature_persistent parameter */
> +	unsigned int feature_persistent_parm:1;
> +	/* Persistent grants feature negotiation result */
>  	unsigned int feature_persistent:1;
>  	unsigned int bounce:1;
>  	unsigned int discard_granularity;
> @@ -1848,7 +1851,7 @@ static int talk_to_blkback(struct xenbus_device *dev,
>  		goto abort_transaction;
>  	}
>  	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
> -			info->feature_persistent);
> +			info->feature_persistent_parm);
>  	if (err)
>  		dev_warn(&dev->dev,
>  			 "writing persistent grants feature to xenbus");
> @@ -2281,7 +2284,8 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
>  	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
>  		blkfront_setup_discard(info);
>  
> -	if (feature_persistent)
> +	info->feature_persistent_parm = feature_persistent;

Same question as before. Why not just use feature_persistent directly?

> +	if (info->feature_persistent_parm)
>  		info->feature_persistent =
>  			!!xenbus_read_unsigned(info->xbdev->otherend,
>  					       "feature-persistent", 0);

Aside: IMO this would look nicer as below:

	info->feature_persistent = feature_persistent && !!xenbus_read_unsigned();