kvm_get_kvm() kvm_put_kvm()

-----Original Message-----
From: Nikita Leshenko [mailto:nikita.leshche...@oracle.com] 
Sent: Tuesday, September 4, 2018 3:57 PM
To: Wu, Fengguang <fengguang...@intel.com>
Cc: Andrew Morton <a...@linux-foundation.org>; Linux Memory Management List 
<linux...@kvack.org>; Peng, DongX <dongx.p...@intel.com>; Liu, Jingqi 
<jingqi....@intel.com>; Dong, Eddie <eddie.d...@intel.com>; Hansen, Dave 
<dave.han...@intel.com>; Huang, Ying <ying.hu...@intel.com>; Brendan Gregg 
<bgr...@netflix.com>; k...@vger.kernel.org; LKML <linux-kernel@vger.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...@intel.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;
> +}

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