Re: [RFC PATCH v4 06/13] vfio: parallelize vfio_pin_map_dma

Daniel Jordan
2018-11-06T02:42:28Z

On Mon, Nov 05, 2018 at 02:51:41PM -0700, Alex Williamson wrote:
> On Mon, 5 Nov 2018 11:55:51 -0500
> Daniel Jordan <daniel.m.jordan@oracle.com> wrote:
> > +static int vfio_pin_map_dma_chunk(unsigned long start_vaddr,
> > + unsigned long end_vaddr,
> > + struct vfio_pin_args *args)
> > {
> > - dma_addr_t iova = dma->iova;
> > - unsigned long vaddr = dma->vaddr;
> > - size_t size = map_size;
> > + struct vfio_dma *dma = args->dma;
> > + dma_addr_t iova = dma->iova + (start_vaddr - dma->vaddr);
> > + unsigned long unmapped_size = end_vaddr - start_vaddr;
> > + unsigned long pfn, mapped_size = 0;
> > long npage;
> > - unsigned long pfn, limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > int ret = 0;
> >
> > - while (size) {
> > + while (unmapped_size) {
> > /* Pin a contiguous chunk of memory */
> > - npage = vfio_pin_pages_remote(dma, vaddr + dma->size,
> > - size >> PAGE_SHIFT, &pfn, limit);
> > + npage = vfio_pin_pages_remote(dma, start_vaddr + mapped_size,
> > + unmapped_size >> PAGE_SHIFT,
> > + &pfn, args->limit, args->mm);
> > if (npage <= 0) {
> > WARN_ON(!npage);
> > ret = (int)npage;
> > @@ -1052,22 +1067,50 @@ static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > }
> >
> > /* Map it! */
> > - ret = vfio_iommu_map(iommu, iova + dma->size, pfn, npage,
> > - dma->prot);
> > + ret = vfio_iommu_map(args->iommu, iova + mapped_size, pfn,
> > + npage, dma->prot);
> > if (ret) {
> > - vfio_unpin_pages_remote(dma, iova + dma->size, pfn,
> > + vfio_unpin_pages_remote(dma, iova + mapped_size, pfn,
> > npage, true);
> > break;
> > }
> >
> > - size -= npage << PAGE_SHIFT;
> > - dma->size += npage << PAGE_SHIFT;
> > + unmapped_size -= npage << PAGE_SHIFT;
> > + mapped_size += npage << PAGE_SHIFT;
> > }
> >
> > + return (ret == 0) ? KTASK_RETURN_SUCCESS : ret;
>
> Overall I'm a big fan of this, but I think there's an undo problem
> here. Per 03/13, kc_undo_func is only called for successfully
> completed chunks and each kc_thread_func should handle cleanup of any
> intermediate work before failure. That's not done here afaict. Should
> we be calling the vfio_pin_map_dma_undo() manually on the completed
> range before returning error?

Yes, we should be, thanks very much for catching this.

At least I documented what I didn't do? :)

>
> > +}
> > +
> > +static void vfio_pin_map_dma_undo(unsigned long start_vaddr,
> > + unsigned long end_vaddr,
> > + struct vfio_pin_args *args)
> > +{
> > + struct vfio_dma *dma = args->dma;
> > + dma_addr_t iova = dma->iova + (start_vaddr - dma->vaddr);
> > + dma_addr_t end = dma->iova + (end_vaddr - dma->vaddr);
> > +
> > + vfio_unmap_unpin(args->iommu, args->dma, iova, end, true);
> > +}
> > +
> > +static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > + size_t map_size)
> > +{
> > + unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > + int ret = 0;
> > + struct vfio_pin_args args = { iommu, dma, limit, current->mm };
> > + /* Stay on PMD boundary in case THP is being used. */
> > + DEFINE_KTASK_CTL(ctl, vfio_pin_map_dma_chunk, &args, PMD_SIZE);
>
> PMD_SIZE chunks almost seems too convenient, I wonder a) is that really
> enough work per thread, and b) is this really successfully influencing
> THP? Thanks,

Yes, you're right on both counts. I'd been using PUD_SIZE for a while in
testing and meant to switch it back to KTASK_MEM_CHUNK (128M) but used PMD_SIZE
by mistake. PUD_SIZE chunks have made thread finishing times too spread out
in some cases, so 128M seems to be a reasonable compromise.

Thanks for the thorough and quick review.

Daniel