Re: [PATCH v7 09/24] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

Alex Williamson
2019-11-21T21:35:48Z

On Wed, 20 Nov 2019 23:13:39 -0800
John Hubbard <jhubbard@nvidia.com> wrote:

> As it says in the updated comment in gup.c: current FOLL_LONGTERM
> behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
> FS DAX check requirement on vmas.
>
> However, the corresponding restriction in get_user_pages_remote() was
> slightly stricter than is actually required: it forbade all
> FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
> that do not set the "locked" arg.
>
> Update the code and comments accordingly, and update the VFIO caller
> to take advantage of this, fixing a bug as a result: the VFIO caller
> is logically a FOLL_LONGTERM user.
>
> Also, remove an unnessary pair of calls that were releasing and
> reacquiring the mmap_sem. There is no need to avoid holding mmap_sem
> just in order to call page_to_pfn().
>
> Also, move the DAX check ("if a VMA is DAX, don't allow long term
> pinning") from the VFIO call site, all the way into the internals
> of get_user_pages_remote() and __gup_longterm_locked(). That is:
> get_user_pages_remote() calls __gup_longterm_locked(), which in turn
> calls check_dax_vmas(). It's lightly explained in the comments as well.
>
> Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
> and to Dan Williams for helping clarify the DAX refactoring.
>
> Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Jerome Glisse <jglisse@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
> drivers/vfio/vfio_iommu_type1.c | 30 +++++-------------------------
> mm/gup.c | 27 ++++++++++++++++++++++-----
> 2 files changed, 27 insertions(+), 30 deletions(-)

Tested with device assignment and Intel mdev vGPU assignment with QEMU
userspace:

Tested-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>

Feel free to include for 19/24 as well. Thanks,

Alex

> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index d864277ea16f..c7a111ad9975 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -340,7 +340,6 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> {
> struct page *page[1];
> struct vm_area_struct *vma;
> - struct vm_area_struct *vmas[1];
> unsigned int flags = 0;
> int ret;
>
> @@ -348,33 +347,14 @@ 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]);
> - }
> - }
> - up_read(&mm->mmap_sem);
> -
> + ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
> + page, NULL, NULL);
> if (ret == 1) {
> *pfn = page_to_pfn(page[0]);
> - return 0;
> + ret = 0;
> + goto done;
> }
>
> - down_read(&mm->mmap_sem);
> -
> vaddr = untagged_addr(vaddr);
>
> vma = find_vma_intersection(mm, vaddr, vaddr + 1);
> @@ -384,7 +364,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> if (is_invalid_reserved_pfn(*pfn))
> ret = 0;
> }
> -
> +done:
> up_read(&mm->mmap_sem);
> return ret;
> }
> diff --git a/mm/gup.c b/mm/gup.c
> index 14fcdc502166..cce2c9676853 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -29,6 +29,13 @@ struct follow_page_context {
> unsigned int page_mask;
> };
>
> +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
> + struct mm_struct *mm,
> + unsigned long start,
> + unsigned long nr_pages,
> + struct page **pages,
> + struct vm_area_struct **vmas,
> + unsigned int flags);
> /*
> * Return the compound head page with ref appropriately incremented,
> * or NULL if that failed.
> @@ -1167,13 +1174,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
> struct vm_area_struct **vmas, int *locked)
> {
> /*
> - * FIXME: Current FOLL_LONGTERM behavior is incompatible with
> + * Parts of FOLL_LONGTERM behavior are incompatible with
> * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
> - * vmas. As there are no users of this flag in this call we simply
> - * disallow this option for now.
> + * vmas. However, this only comes up if locked is set, and there are
> + * callers that do request FOLL_LONGTERM, but do not set locked. So,
> + * allow what we can.
> */
> - if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> - return -EINVAL;
> + if (gup_flags & FOLL_LONGTERM) {
> + if (WARN_ON_ONCE(locked))
> + return -EINVAL;
> + /*
> + * This will check the vmas (even if our vmas arg is NULL)
> + * and return -ENOTSUPP if DAX isn't allowed in this case:
> + */
> + return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
> + vmas, gup_flags | FOLL_TOUCH |
> + FOLL_REMOTE);
> + }
>
> return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
> locked,