linux-ext4 - Re: [PATCH v4 07/13] lib: test_hmm add ioctl to get zone device type

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Date:   Tue, 31 May 2022 10:31:52 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alex Sierra <alex.sierra@....com>
Cc:     <jgg@...dia.com>, <david@...hat.com>, <Felix.Kuehling@....com>,
        <linux-mm@...ck.org>, <rcampbell@...dia.com>,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        <amd-gfx@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
        <hch@....de>, <jglisse@...hat.com>, <apopple@...dia.com>,
        <willy@...radead.org>
Subject: Re: [PATCH v4 07/13] lib: test_hmm add ioctl to get zone device
 type

On Tue, 31 May 2022 10:56:23 -0500 Alex Sierra <alex.sierra@....com> wrote:

> new ioctl cmd added to query zone device type. This will be
> used once the test_hmm adds zone device coherent type.
> 
> @@ -1026,6 +1027,15 @@ static int dmirror_snapshot(struct dmirror *dmirror,
>  	return ret;
>  }
>  
> +static int dmirror_get_device_type(struct dmirror *dmirror,
> +			    struct hmm_dmirror_cmd *cmd)
> +{
> +	mutex_lock(&dmirror->mutex);
> +	cmd->zone_device_type = dmirror->mdevice->zone_device_type;
> +	mutex_unlock(&dmirror->mutex);

What does the locking here do?

Presumably cmd->zone_device_type can become out of date the instant the
mutex is released, so what was the point in taking the mutex?

And does it make sense to return potentially out-of-date info to
userspace?  Perhaps this interface simply shouldn't exist?