linux-kernel - Re: [RFC][PATCH 3/5] [PATCH 3/5] kvm-ept-idle: HVA indexed EPT read

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Date:   Tue, 4 Sep 2018 09:57:01 +0200
From:   Nikita Leshenko <nikita.leshchenko@...cle.com>
To:     Fengguang Wu <fengguang.wu@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Peng DongX <dongx.peng@...el.com>,
        Liu Jingqi <jingqi.liu@...el.com>,
        Dong Eddie <eddie.dong@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Huang Ying <ying.huang@...el.com>,
        Brendan Gregg <bgregg@...flix.com>, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 3/5] [PATCH 3/5] kvm-ept-idle: HVA indexed EPT read

On 1 Sep 2018, at 13:28, Fengguang Wu <fengguang.wu@...el.com> wrote:
> +static ssize_t ept_idle_read(struct file *file, char *buf,
> +			     size_t count, loff_t *ppos)
> +{
> +	struct task_struct *task = file->private_data;
> +	struct ept_idle_ctrl *eic;
> +	unsigned long hva_start = *ppos << BITMAP_BYTE2PVA_SHIFT;
> +	unsigned long hva_end = hva_start + (count << BITMAP_BYTE2PVA_SHIFT);
> +	int ret;
> +
> +	if (*ppos % IDLE_BITMAP_CHUNK_SIZE ||
> +	    count % IDLE_BITMAP_CHUNK_SIZE)
> +		return -EINVAL;
> +
> +	eic = kzalloc(sizeof(*eic), GFP_KERNEL);
> +	if (!eic)
> +		return -EBUSY;
> +
> +	eic->buf = buf;
> +	eic->buf_size = count;
> +	eic->kvm = task_kvm(task);
> +	if (!eic->kvm) {
> +		ret = -EINVAL;
> +		goto out_free;
> +	}
I think you need to increment the refcount while using kvm,
otherwise kvm can be destroyed from another thread while you're
walking it.

-Nikita
> +
> +	ret = ept_idle_walk_hva_range(eic, hva_start, hva_end);
> +	if (ret)
> +		goto out_free;
> +
> +	ret = eic->bytes_copied;
> +	*ppos += ret;
> +out_free:
> +	kfree(eic);
> +
> +	return ret;
> +}