On Mon, Oct 21, 2019 at 01:54:15PM -0700, Ralph Campbell wrote:
>
> On 10/21/19 11:49 AM, Jerome Glisse wrote:
> > On Tue, Oct 15, 2019 at 01:48:13PM -0700, Ralph Campbell wrote:
> > > Allow hmm_range_fault() to return success (0) when the CPU pagetable
> > > entry points to the special shared zero page.
> > > The caller can then handle the zero page by possibly clearing device
> > > private memory instead of DMAing a zero page.
> >
> > I do not understand why you are talking about DMA. GPU can work
> > on main memory and migrating to GPU memory is optional and should
> > not involve this function at all.
>
> Good point. This is the device accessing the zero page over PCIe
> or another bus, not migrating a zero page to device private memory.
> I'll update the wording.
>
> > >
> > > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > > Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> >
> > NAK please keep semantic or change it fully. See the alternative
> > below.
> >
> > > ---
> > > mm/hmm.c | 4 +++-
> > > 1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/hmm.c b/mm/hmm.c
> > > index 5df0dbf77e89..f62b119722a3 100644
> > > --- a/mm/hmm.c
> > > +++ b/mm/hmm.c
> > > @@ -530,7 +530,9 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> > > return -EBUSY;
> > > } else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> > > *pfn = range->values[HMM_PFN_SPECIAL];
> > > - return -EFAULT;
> > > + if (!is_zero_pfn(pte_pfn(pte)))
> > > + return -EFAULT;
> > > + return 0;
> >
> > An acceptable change would be to turn the branch into:
> > } else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> > if (!is_zero_pfn(pte_pfn(pte))) {
> > *pfn = range->values[HMM_PFN_SPECIAL];
> > return -EFAULT;
> > }
> > /* Fall-through for zero pfn (if write was needed the above
> > * hmm_pte_need_faul() would had catched it).
> > */
> > }
> >
>
> Except this will return the zero pfn with no indication that it is special
> (i.e., doesn't have a struct page).
That is fine, the device driver should not do anything with it ie
if the device driver wanted to write then the write fault test
would return true and it would fault.
Note that driver should not dereference the struct page.
Cheers,
Jérôme
