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

John Hubbard
2019-11-12T22:24:45Z

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,
--
John Hubbard
NVIDIA