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

[Ira Weiny]

On Wed, Nov 13, 2019 at 09:02:02AM -0400, Jason Gunthorpe wrote:
> 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

Ah yea...  retract my reviewed by...  :-(

Ira