linux-kernel - Re: [PATCH v13 11/22] vfio iommu: Add blocking notifier to notify DMA

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Date:   Tue, 15 Nov 2016 20:16:12 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <kevin.tian@...el.com>, <jike.song@...el.com>,
        <bjsdjshi@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v13 11/22] vfio iommu: Add blocking notifier to notify
 DMA_UNMAP

On Wed, 16 Nov 2016 08:16:15 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 11/16/2016 3:49 AM, Alex Williamson wrote:
> > On Tue, 15 Nov 2016 20:59:54 +0530
> > Kirti Wankhede <kwankhede@...dia.com> wrote:
> >   
> ...
> 
> >> @@ -854,7 +857,28 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >>  		 */
> >>  		if (dma->task->mm != current->mm)
> >>  			break;
> >> +
> >>  		unmapped += dma->size;
> >> +
> >> +		if (iommu->external_domain && !RB_EMPTY_ROOT(&dma->pfn_list)) {
> >> +			struct vfio_iommu_type1_dma_unmap nb_unmap;
> >> +
> >> +			nb_unmap.iova = dma->iova;
> >> +			nb_unmap.size = dma->size;
> >> +
> >> +			/*
> >> +			 * Notifier callback would call vfio_unpin_pages() which
> >> +			 * would acquire iommu->lock. Release lock here and
> >> +			 * reacquire it again.
> >> +			 */
> >> +			mutex_unlock(&iommu->lock);
> >> +			blocking_notifier_call_chain(&iommu->notifier,
> >> +						    VFIO_IOMMU_NOTIFY_DMA_UNMAP,
> >> +						    &nb_unmap);
> >> +			mutex_lock(&iommu->lock);
> >> +			if (WARN_ON(!RB_EMPTY_ROOT(&dma->pfn_list)))
> >> +				break;
> >> +		}  
> > 
> > 
> > Why exactly do we need to notify per vfio_dma rather than per unmap
> > request?  If we do the latter we can send the notify first, limiting us
> > to races where a page is pinned between the notify and the locking,
> > whereas here, even our dma pointer is suspect once we re-acquire the
> > lock, we don't technically know if another unmap could have removed
> > that already.  Perhaps something like this (untested):
> >   
> 
> There are checks to validate unmap request, like v2 check and who is
> calling unmap and is it allowed for that task to unmap. Before these
> checks its not sure that unmap region range which asked for would be
> unmapped all. Notify call should be at the place where its sure that the
> range provided to notify call is definitely going to be removed. My
> change do that.

Ok, but that does solve the problem.  What about this (untested):

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index ee9a680..50cafdf 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -782,9 +782,9 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 			     struct vfio_iommu_type1_dma_unmap *unmap)
 {
 	uint64_t mask;
-	struct vfio_dma *dma;
+	struct vfio_dma *dma, *dma_last = NULL;
 	size_t unmapped = 0;
-	int ret = 0;
+	int ret = 0, retries;
 
 	mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
 
@@ -794,7 +794,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 		return -EINVAL;
 
 	WARN_ON(mask & PAGE_MASK);
-
+again:
 	mutex_lock(&iommu->lock);
 
 	/*
@@ -851,11 +851,16 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 		if (dma->task->mm != current->mm)
 			break;
 
-		unmapped += dma->size;
-
-		if (iommu->external_domain && !RB_EMPTY_ROOT(&dma->pfn_list)) {
+		if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
 			struct vfio_iommu_type1_dma_unmap nb_unmap;
 
+			if (dma_last == dma) {
+				BUG_ON(++retries > 10);
+			} else {
+				dma_last = dma;
+				retries = 0;
+			}
+
 			nb_unmap.iova = dma->iova;
 			nb_unmap.size = dma->size;
 
@@ -868,11 +873,11 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 			blocking_notifier_call_chain(&iommu->notifier,
 						    VFIO_IOMMU_NOTIFY_DMA_UNMAP,
 						    &nb_unmap);
-			mutex_lock(&iommu->lock);
-			if (WARN_ON(!RB_EMPTY_ROOT(&dma->pfn_list)))
-				break;
+			goto again:
 		}
+		unmapped += dma->size;
 		vfio_remove_dma(iommu, dma);
+
 	}
 
 unlock: