Re: [PATCH v11 06/25] mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Dec 16, 2019 at 02:25:18PM -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 to loosen the restriction, allowing
> FOLL_LONGTERM in some cases.
> 
> Also, copy 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(). This check will then be removed from the VFIO
> call site in a subsequent patch.
> 
> Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
> and to Dan Williams for helping clarify the DAX refactoring.
> 
> Tested-by: Alex Williamson <alex.williamson@redhat.com>
> Acked-by: Alex Williamson <alex.williamson@redhat.com>
> 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>
> ---
>  mm/gup.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 3ecce297a47f..c0c56888e7cc 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);

Any particular reason for the forward declaration? Maybe move
get_user_pages_remote() down?

>  /*
>   * Return the compound head page with ref appropriately incremented,
>   * or NULL if that failed.
> @@ -1179,13 +1186,23 @@ 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,
> -- 
> 2.24.1
> 

-- 
 Kirill A. Shutemov