linux-kernel - Re: [PATCH v4 1/2] vfio/type1: Remove locked page accounting workqueue

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Date:   Mon, 17 Apr 2017 14:47:54 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, eric.auger@...hat.com, kwankhede@...dia.com,
        linux-kernel@...r.kernel.org, slp@...hat.com
Subject: Re: [PATCH v4 1/2] vfio/type1: Remove locked page accounting
 workqueue

On Sun, Apr 16, 2017 at 07:42:27PM -0600, Alex Williamson wrote:

[...]

> -static void vfio_lock_acct(struct task_struct *task, long npage)
> +static int vfio_lock_acct(struct task_struct *task, long npage, bool lock_cap)
>  {
> -	struct vwork *vwork;
>  	struct mm_struct *mm;
>  	bool is_current;
> +	int ret;
>  
>  	if (!npage)
> -		return;
> +		return 0;
>  
>  	is_current = (task->mm == current->mm);
>  
>  	mm = is_current ? task->mm : get_task_mm(task);
>  	if (!mm)
> -		return; /* process exited */
> +		return -ESRCH; /* process exited */
>  
> -	if (down_write_trylock(&mm->mmap_sem)) {
> -		mm->locked_vm += npage;
> -		up_write(&mm->mmap_sem);
> -		if (!is_current)
> -			mmput(mm);
> -		return;
> -	}
> +	ret = down_write_killable(&mm->mmap_sem);
> +	if (!ret) {
> +		if (npage < 0 || lock_cap) {

Nit: maybe we can avoid passing in lock_cap in all the callers of
vfio_lock_acct() and fetch it via has_capability() only if npage < 0?
IMHO that'll keep the vfio_lock_acct() interface cleaner, and we won't
need to pass in "false" any time when doing unpins.

[...]

> @@ -405,7 +379,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  				  long npage, unsigned long *pfn_base)
>  {
> -	unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +	unsigned long pfn = 0, limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>  	bool lock_cap = capable(CAP_IPC_LOCK);
>  	long ret, pinned = 0, lock_acct = 0;
>  	bool rsvd;
> @@ -442,8 +416,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  	/* Lock all the consecutive pages from pfn_base */
>  	for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; pinned < npage;
>  	     pinned++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
> -		unsigned long pfn = 0;
> -
>  		ret = vaddr_get_pfn(current->mm, vaddr, dma->prot, &pfn);
>  		if (ret)
>  			break;
> @@ -460,14 +432,25 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  				put_pfn(pfn, dma->prot);
>  				pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
>  					__func__, limit << PAGE_SHIFT);
> -				break;
> +				ret = -ENOMEM;
> +				goto unpin_out;
>  			}
>  			lock_acct++;
>  		}
>  	}
>  
>  out:
> -	vfio_lock_acct(current, lock_acct);
> +	ret = vfio_lock_acct(current, lock_acct, lock_cap);

I just didn't notice this in previous review, but... do we need to
check against !rsvd as well here before doing the accounting?

Thanks!

> +
> +unpin_out:
> +	if (ret) {
> +		if (!rsvd) {
> +			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> +				put_pfn(pfn, dma->prot);
> +		}
> +
> +		return ret;
> +	}
>  
>  	return pinned;
>  }

-- 
Peter Xu