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Re: [Qemu-devel] [PATCH] vfio: If DMA map returns ENOMEM wait and try ag
From: |
Alex Williamson |
Subject: |
Re: [Qemu-devel] [PATCH] vfio: If DMA map returns ENOMEM wait and try again |
Date: |
Mon, 3 Apr 2017 09:40:15 -0600 |
On Mon, 3 Apr 2017 10:58:22 +0200
Sergio Lopez <address@hidden> wrote:
> When quickly unmapping and mapping memory regions (as may happen in
> address_space_update_topology), if running with a non-unlimited
> RLIMIT_MEMLOCK, the kernel may return ENOMEM for a map request
> because the previous unmap has been processed, but accounted yet.
>
> Probably this should be fixed in the kernel ensuring a deterministic
> behavior for VFIO map and unmap operations. Until then, this works
> around the issue, waiting 10ms and trying again.
I think we need to know what that kernel fix is before adding arbitrary
delays and retries in userspace code (Do we know why 10ms works? Is
it too long/short?). I think I have a test program that reproduces
this, I setup vfio and allocate two 4k buffers, one for mapping through
vfio and one for mlocking. I clone(2) the process with CLONE_VM and the
clone loops doing mlock/munlock while the main thread does map/unmap.
This fails in a fraction of a second while running either independently
works well. Still investigating. Thanks,
Alex
> Signed-off-by: Sergio Lopez <address@hidden>
> ---
> hw/vfio/common.c | 31 +++++++++++++++++++++++--------
> 1 file changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index f3ba9b9..db41fa5 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -228,17 +228,32 @@ static int vfio_dma_map(VFIOContainer *container,
> hwaddr iova,
> map.flags |= VFIO_DMA_MAP_FLAG_WRITE;
> }
>
> - /*
> - * Try the mapping, if it fails with EBUSY, unmap the region and try
> - * again. This shouldn't be necessary, but we sometimes see it in
> - * the VGA ROM space.
> - */
> - if (ioctl(container->fd, VFIO_IOMMU_MAP_DMA, &map) == 0 ||
> - (errno == EBUSY && vfio_dma_unmap(container, iova, size) == 0 &&
> - ioctl(container->fd, VFIO_IOMMU_MAP_DMA, &map) == 0)) {
> + if (ioctl(container->fd, VFIO_IOMMU_MAP_DMA, &map) == 0) {
> return 0;
> }
>
> + if (errno == ENOMEM) {
> + /*
> + * When quickly unmapping and mapping ranges, the kernel may
> + * return ENOMEM for a map request because the previous unmap
> + * has not been accounted yet. Wait a bit and try again.
> + */
> + usleep(10 * 1000);
> + if (ioctl(container->fd, VFIO_IOMMU_MAP_DMA, &map) == 0) {
> + return 0;
> + }
> + } else if (errno == EBUSY) {
> + /*
> + * If mapping fails with EBUSY, unmap the region and try again.
> + * This shouldn't be necessary, but we sometimes see it in the
> + * VGA ROM space.
> + */
> + if (vfio_dma_unmap(container, iova, size) == 0 &&
> + ioctl(container->fd, VFIO_IOMMU_MAP_DMA, &map) == 0) {
> + return 0;
> + }
> + }
> +
> error_report("VFIO_MAP_DMA: %d", -errno);
> return -errno;
> }