Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

On 11/12/19 1:57 PM, Dan Williams wrote: ...

...

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index d864277ea16f..017689b7c32b 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -348,24 +348,20 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, flags |= FOLL_WRITE;

    down_read(&mm->mmap_sem);

•   if (mm == current->mm) {
  

•           ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
  

•                                vmas);
  

•   } else {
  

•           ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
  

•                                       vmas, NULL);
  

•           /*
  

•            * The lifetime of a vaddr_get_pfn() page pin is
  

•            * userspace-controlled. In the fs-dax case this could
  

•            * lead to indefinite stalls in filesystem operations.
  

•            * Disallow attempts to pin fs-dax pages via this
  

•            * interface.
  

•            */
  

•           if (ret > 0 && vma_is_fsdax(vmas[0])) {
  

•                   ret = -EOPNOTSUPP;
  

•                   put_page(page[0]);
  

•           }
  

•   ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
  

•                               page, vmas, NULL);
  

Hmm, what's the point of passing FOLL_LONGTERM to get_user_pages_remote() if get_user_pages_remote() is not going to check the vma? I think we got to this code state because the

FOLL_LONGTERM is short-lived in this location, because patch 23 ("mm/gup: remove support for gup(FOLL_LONGTERM)") removes it, after callers are changed over to pin_longterm_pages*().

So FOLL_LONGTERM is not doing much now, but it is basically a marker for "change gup(FOLL_LONGTERM) to pin_longterm_pages()", and patch 18 actually makes that change.

And then pin_longterm_pages*() is, in turn, a way to mark all the places that need file system and/or user space interactions (layout leases, etc), as per "Case 2: RDMA" in the new Documentation/vm/pin_user_pages.rst.

get_user_pages() vs get_user_pages_remote() split predated the introduction of FOLL_LONGTERM.

Yes. And I do want clean this up as I go, so we don't end up with stale concepts lingering in gup.c...

I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) && vma_is_fsdax()) check and that would also remove the need for __gup_longterm_locked.

Good idea, but there is still the call to check_and_migrate_cma_pages(), inside __gup_longterm_locked(). So it's a little more involved and we can't trivially delete __gup_longterm_locked() yet, right?

thanks,