Re: [PATCH v2] vfio/type1: Remove locked page accounting workqueue

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On Thu, 6 Apr 2017 18:53:04 +0200
Auger Eric <eric.auger@xxxxxxxxxx> wrote:

> Hi Alex,
> 
> On 06/04/2017 16:53, Alex Williamson wrote:
> > If the mmap_sem is contented then the vfio type1 IOMMU backend will
> > defer locked page accounting updates to a workqueue task.  This has
> > a few problems and depending on which side the user tries to play,
> > they might be over-penalized for unmaps that haven't yet been
> > accounted, or able to race the workqueue to enter more mappings
> > than they're allowed.  It's not entirely clear what motivated this
> > workqueue mechanism in the original vfio design, but it seems to
> > introduce more problems than it solves, so remove it and update the
> > callers to allow for failure.  We can also now recheck the limit
> > under write lock to make sure we don't exceed it.
> > 
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>  
> Looks good to me.
> 
> Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>
> 

One possible enhancement below.

> 
> > ---
> > 
> > v2: Fixed missed mmput on failure to acquire mmap_sem as noted by Eric
> > 
> >  drivers/vfio/vfio_iommu_type1.c |  101 ++++++++++++++++++---------------------
> >  1 file changed, 46 insertions(+), 55 deletions(-)
> > 
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 32d2633092a3..b799edbb8c4f 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -246,69 +246,45 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
> >  	return ret;
> >  }
> >  
> > -struct vwork {
> > -	struct mm_struct	*mm;
> > -	long			npage;
> > -	struct work_struct	work;
> > -};
> > -
> > -/* delayed decrement/increment for locked_vm */
> > -static void vfio_lock_acct_bg(struct work_struct *work)
> > +static int vfio_lock_acct(struct task_struct *task, long npage)
> >  {
> > -	struct vwork *vwork = container_of(work, struct vwork, work);
> > -	struct mm_struct *mm;
> > -
> > -	mm = vwork->mm;
> > -	down_write(&mm->mmap_sem);
> > -	mm->locked_vm += vwork->npage;
> > -	up_write(&mm->mmap_sem);
> > -	mmput(mm);
> > -	kfree(vwork);
> > -}
> > -
> > -static void vfio_lock_acct(struct task_struct *task, long npage)
> > -{
> > -	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);


Hmm, since we're already testing current, I wonder if it makes sense to
have this bimodal, killable if current, straight down_write()
otherwise.  I'm not too sure how important it is to use killable
regardless, but mlock does, which seemed like a good model to follow.
Thanks,

Alex

> > +	if (!ret) {
> > +		if (npage < 0) {
> > +			mm->locked_vm += npage;
> > +		} else {
> > +			unsigned long limit;
> > +
> > +			limit = is_current ?
> > +				rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT :
> > +				task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > +
> > +			if (mm->locked_vm + npage <= limit)
> > +				mm->locked_vm += npage;
> > +			else
> > +				ret = -ENOMEM;
> > +		}
> >  
> > -	if (is_current) {
> > -		mm = get_task_mm(task);
> > -		if (!mm)
> > -			return;
> > +		up_write(&mm->mmap_sem);
> >  	}
> >  
> > -	/*
> > -	 * Couldn't get mmap_sem lock, so must setup to update
> > -	 * mm->locked_vm later. If locked_vm were atomic, we
> > -	 * wouldn't need this silliness
> > -	 */
> > -	vwork = kmalloc(sizeof(struct vwork), GFP_KERNEL);
> > -	if (WARN_ON(!vwork)) {
> > +	if (!is_current)
> >  		mmput(mm);
> > -		return;
> > -	}
> > -	INIT_WORK(&vwork->work, vfio_lock_acct_bg);
> > -	vwork->mm = mm;
> > -	vwork->npage = npage;
> > -	schedule_work(&vwork->work);
> > +
> > +	return ret;
> >  }
> >  
> >  /*
> > @@ -405,7 +381,7 @@ 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 +418,6 @@ 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 +434,25 @@ 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);
> > +
> > +unpin_out:
> > +	if (ret) {
> > +		if (!rsvd) {
> > +			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> > +				put_pfn(pfn, dma->prot);
> > +		}
> > +
> > +		return ret;
> > +	}
> >  
> >  	return pinned;
> >  }
> > @@ -522,8 +507,14 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> >  		goto pin_page_exit;
> >  	}
> >  
> > -	if (!rsvd && do_accounting)
> > -		vfio_lock_acct(dma->task, 1);
> > +	if (!rsvd && do_accounting) {
> > +		ret = vfio_lock_acct(dma->task, 1);
> > +		if (ret) {
> > +			put_pfn(*pfn_base, dma->prot);
> > +			goto pin_page_exit;
> > +		}
> > +	}
> > +
> >  	ret = 1;
> >  
> >  pin_page_exit:
> >   




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