[PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk

[Dan Williams]

On Thu, Aug 15, 2019 at 1:41 PM Jason Gunthorpe <jgg at mellanox.com> wrote:
>
> On Thu, Aug 15, 2019 at 04:33:06PM -0400, Jerome Glisse wrote:
>
> > So nor HMM nor driver should dereference the struct page (i do not
> > think any iommu driver would either),
>
> Er, they do technically deref the struct page:
>
> nouveau_dmem_convert_pfn(struct nouveau_drm *drm,
>                          struct hmm_range *range)
>                 struct page *page;
>                 page = hmm_pfn_to_page(range, range->pfns[i]);
>                 if (!nouveau_dmem_page(drm, page)) {
>
>
> nouveau_dmem_page(struct nouveau_drm *drm, struct page *page)
> {
>         return is_device_private_page(page) && drm->dmem == page_to_dmem(page)
>
>
> Which does touch 'page->pgmap'
>
> Is this OK without having a get_dev_pagemap() ?
>
> Noting that the collision-retry scheme doesn't protect anything here
> as we can have a concurrent invalidation while doing the above deref.

As long take_driver_page_table_lock() in Jerome's flow can replace
percpu_ref_tryget_live() on the pagemap reference. It seems
nouveau_dmem_convert_pfn() happens after:

                        mutex_lock(&svmm->mutex);
                        if (!nouveau_range_done(&range)) {

...so I would expect that to be functionally equivalent to validating
the reference count.