Re: [PATCH 6/9] iommu/dma-iommu.c: Convert to use vm_insert_range

[Robin Murphy]

On 26/11/2018 06:44, Souptick Joarder wrote:
> On Sat, Nov 24, 2018 at 3:04 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > On Fri, Nov 23, 2018 at 05:23:06PM +0000, Robin Murphy wrote:
> > > On 15/11/2018 15:49, Souptick Joarder wrote:
> > > > Convert to use vm_insert_range() to map range of kernel
> > > > memory to user vma.
> > > >
> > > > Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> > > > Reviewed-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > > ---
> > > >    drivers/iommu/dma-iommu.c | 12 ++----------
> > > >    1 file changed, 2 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > > > index d1b0475..69c66b1 100644
> > > > --- a/drivers/iommu/dma-iommu.c
> > > > +++ b/drivers/iommu/dma-iommu.c
> > > > @@ -622,17 +622,9 @@ struct page **iommu_dma_alloc(struct device *dev, size_t size, gfp_t gfp,
> > > >    int iommu_dma_mmap(struct page **pages, size_t size, struct vm_area_struct *vma)
> > > >    {
> > > > -   unsigned long uaddr = vma->vm_start;
> > > > -   unsigned int i, count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> > > > -   int ret = -ENXIO;
> > > > +   unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> > > > -   for (i = vma->vm_pgoff; i < count && uaddr < vma->vm_end; i++) {
> > > > -           ret = vm_insert_page(vma, uaddr, pages[i]);
> > > > -           if (ret)
> > > > -                   break;
> > > > -           uaddr += PAGE_SIZE;
> > > > -   }
> > > > -   return ret;
> > > > +   return vm_insert_range(vma, vma->vm_start, pages, count);
> > >
> > > AFIACS, vm_insert_range() doesn't respect vma->vm_pgoff, so doesn't this
> > > break partial mmap()s of a large buffer? (which I believe can be a thing)
> >
> > Whoops.  That should have been:
> >
> > return vm_insert_range(vma, vma->vm_start, pages + vma->vm_pgoff, count);
>
> I am unable to trace back where vma->vm_pgoff is set for this driver ? if any ?

This isn't a driver - it's a DMA API backend, so any caller of 
dma_mmap_*() could potentially end up here (similarly for patch 2/9).

Robin.

> If default value set to 0 then I think existing code is correct.
>
> > I suppose.
>
> > Although arguably we should respect vm_pgoff inside vm_insert_region()
> > and then callers automatically get support for vm_pgoff without having
> > to think about it ...
>
> I assume, vm_insert_region() means vm_insert_range(). If we respect vm_pgoff
> inside vm_insert_range, for any uninitialized/ error value set for vm_pgoff from
> drivers will introduce a bug inside core mm which might be difficult
> to trace back.
> But when vm_pgoff set and passed from caller (drivers) it might be
> easy to figure out.
>
> > although we should then also pass in the length
> > of the pages array to avoid pages being mapped in which aren't part of
> > the allocated array.
>
> Mostly Partial mapping is done by starting from an index and mapped it till
> end of pages array. Calculating length of the pages array will have a small
> overhead for each drivers.
>
> Please correct me if I am wrong.