Re: [PATCH v4 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

[Jason Gunthorpe]

On Tue, Nov 12, 2019 at 08:26:55PM -0800, John Hubbard 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.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
>  drivers/vfio/vfio_iommu_type1.c | 25 ++-----------------------
>  mm/gup.c                        | 27 ++++++++++++++++++++++-----
>  2 files changed, 24 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index d864277ea16f..7301b710c9a4 100644
> +++ 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 +347,13 @@ 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;

Mind the return with the lock held this needs some goto unwind

Jason