Subject: Re: [PATCH kernel v11 09/34] vfio: powerpc/spapr: Move locked_vm accounting to helpers

From: David Gibson <[email-masked]>
Date: Thu, 4 Jun 2015 11:00:28 +1000

On Wed, Jun 03, 2015 at 09:11:09PM +1000, Alexey Kardashevskiy wrote:
> On 06/01/2015 02:28 PM, David Gibson wrote:
> >On Fri, May 29, 2015 at 06:44:33PM +1000, Alexey Kardashevskiy wrote:
> >>There moves locked pages accounting to helpers.
> >>Later they will be reused for Dynamic DMA windows (DDW).
> >>
> >>This reworks debug messages to show the current value and the limit.
> >>
> >>This stores the locked pages number in the container so when unlocking
> >>the iommu table pointer won't be needed. This does not have an effect
> >>now but it will with the multiple tables per container as then we will
> >>allow attaching/detaching groups on fly and we may end up having
> >>a container with no group attached but with the counter incremented.
> >>
> >>While we are here, update the comment explaining why RLIMIT_MEMLOCK
> >>might be required to be bigger than the guest RAM. This also prints
> >>pid of the current process in pr_warn/pr_debug.
> >>
> >>Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> >>[aw: for the vfio related changes]
> >>Acked-by: Alex Williamson <[email-masked]>
> >>Reviewed-by: David Gibson <[email-masked]>
> >>Reviewed-by: Gavin Shan <[email-masked]>
> >>---
> >>Changes:
> >>v4:
> >>* new helpers do nothing if @npages == 0
> >>* tce_iommu_disable() now can decrement the counter if the group was
> >>detached (not possible now but will be in the future)
> >>---
> >>  drivers/vfio/vfio_iommu_spapr_tce.c | 82 ++++++++++++++++++++++++++++---------
> >>  1 file changed, 63 insertions(+), 19 deletions(-)
> >>
> >>diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>index 64300cc..40583f9 100644
> >>--- a/drivers/vfio/vfio_iommu_spapr_tce.c
> >>+++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>@@ -29,6 +29,51 @@
> >>  static void tce_iommu_detach_group(void *iommu_data,
> >>  		struct iommu_group *iommu_group);
> >>
> >>+static long try_increment_locked_vm(long npages)
> >>+{
> >>+	long ret = 0, locked, lock_limit;
> >>+
> >>+	if (!current || !current->mm)
> >>+		return -ESRCH; /* process exited */
> >>+
> >>+	if (!npages)
> >>+		return 0;
> >>+
> >>+	down_write(&current->mm->mmap_sem);
> >>+	locked = current->mm->locked_vm + npages;
> >
> >Is there a possibility of userspace triggering an integer overflow
> >here, if npages is really huge?
> 
> 
> I do not see how. I just do not accept npages bigger than the host RAM size
> in pages. And it is "long". For (lets say) 128GB host, the number of 4KB
> pages is (128<<30)/4096=33554432.

Ah, yes, npages has already been shifted right so it should be safe. Ok.

> 
> 
> >
> >>+	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> >>+	if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> >>+		ret = -ENOMEM;
> >>+	else
> >>+		current->mm->locked_vm += npages;
> >>+
> >>+	pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%ld%s\n", current->pid,
> >>+			npages << PAGE_SHIFT,
> >>+			current->mm->locked_vm << PAGE_SHIFT,
> >>+			rlimit(RLIMIT_MEMLOCK),
> >>+			ret ? " - exceeded" : "");
> >>+
> >>+	up_write(&current->mm->mmap_sem);
> >>+
> >>+	return ret;
> >>+}
> >>+
> >>+static void decrement_locked_vm(long npages)
> >>+{
> >>+	if (!current || !current->mm || !npages)
> >>+		return; /* process exited */
> >>+
> >>+	down_write(&current->mm->mmap_sem);
> >>+	if (npages > current->mm->locked_vm)
> >>+		npages = current->mm->locked_vm;
> >
> >Can this case ever occur (without there being a leak bug somewhere
> >else in the code)?
> 
> 
> It should not. Safety measure. Having a warning here might make sense but I
> believe if this happens, there will be many, many warnings in other places
> :)

Ok.  I'd would be nice to see a WARN_ON() as documentation that this
isn't a situation that should ever happen.  I wouldn't nack on that
basis alone though.

> >>+	current->mm->locked_vm -= npages;
> >>+	pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%ld\n", current->pid,
> >>+			npages << PAGE_SHIFT,
> >>+			current->mm->locked_vm << PAGE_SHIFT,
> >>+			rlimit(RLIMIT_MEMLOCK));
> >>+	up_write(&current->mm->mmap_sem);
> >>+}
> >>+
> >>  /*
> >>   * VFIO IOMMU fd for SPAPR_TCE IOMMU implementation
> >>   *
> >>@@ -45,6 +90,7 @@ struct tce_container {
> >>  	struct mutex lock;
> >>  	struct iommu_table *tbl;
> >>  	bool enabled;
> >>+	unsigned long locked_pages;
> >>  };
> >>
> >>  static bool tce_page_is_contained(struct page *page, unsigned page_shift)
> >>@@ -60,7 +106,7 @@ static bool tce_page_is_contained(struct page *page, unsigned page_shift)
> >>  static int tce_iommu_enable(struct tce_container *container)
> >>  {
> >>  	int ret = 0;
> >>-	unsigned long locked, lock_limit, npages;
> >>+	unsigned long locked;
> >>  	struct iommu_table *tbl = container->tbl;
> >>
> >>  	if (!container->tbl)
> >>@@ -89,21 +135,22 @@ static int tce_iommu_enable(struct tce_container *container)
> >>  	 * Also we don't have a nice way to fail on H_PUT_TCE due to ulimits,
> >>  	 * that would effectively kill the guest at random points, much better
> >>  	 * enforcing the limit based on the max that the guest can map.
> >>+	 *
> >>+	 * Unfortunately at the moment it counts whole tables, no matter how
> >>+	 * much memory the guest has. I.e. for 4GB guest and 4 IOMMU groups
> >>+	 * each with 2GB DMA window, 8GB will be counted here. The reason for
> >>+	 * this is that we cannot tell here the amount of RAM used by the guest
> >>+	 * as this information is only available from KVM and VFIO is
> >>+	 * KVM agnostic.
> >>  	 */
> >>-	down_write(&current->mm->mmap_sem);
> >>-	npages = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
> >>-	locked = current->mm->locked_vm + npages;
> >>-	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> >>-	if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
> >>-		pr_warn("RLIMIT_MEMLOCK (%ld) exceeded\n",
> >>-				rlimit(RLIMIT_MEMLOCK));
> >>-		ret = -ENOMEM;
> >>-	} else {
> >>+	locked = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
> >>+	ret = try_increment_locked_vm(locked);
> >>+	if (ret)
> >>+		return ret;
> >>
> >>-		current->mm->locked_vm += npages;
> >>-		container->enabled = true;
> >>-	}
> >>-	up_write(&current->mm->mmap_sem);
> >>+	container->locked_pages = locked;
> >>+
> >>+	container->enabled = true;
> >>
> >>  	return ret;
> >>  }
> >>@@ -115,13 +162,10 @@ static void tce_iommu_disable(struct tce_container *container)
> >>
> >>  	container->enabled = false;
> >>
> >>-	if (!container->tbl || !current->mm)
> >>+	if (!current->mm)
> >>  		return;
> >>
> >>-	down_write(&current->mm->mmap_sem);
> >>-	current->mm->locked_vm -= (container->tbl->it_size <<
> >>-			container->tbl->it_page_shift) >> PAGE_SHIFT;
> >>-	up_write(&current->mm->mmap_sem);
> >>+	decrement_locked_vm(container->locked_pages);
> >>  }
> >>
> >>  static void *tce_iommu_open(unsigned long arg)
> >
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson