Re: [PATCH hmm 1/8] mm/hmm: add missing unmaps of the ptep during hmm_vma_handle_pte()

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Mar 11, 2020 at 06:28:30PM -0700, Ralph Campbell wrote:
> >   mm/hmm.c | 8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 72e5a6d9a41756..35f85424176d14 100644
> > +++ b/mm/hmm.c
> > @@ -325,6 +325,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> >   		}
> >   		/* Report error for everything else */
> > +		pte_unmap(ptep);
> >   		*pfn = range->values[HMM_PFN_ERROR];
> >   		return -EFAULT;
> >   	} else {
> > @@ -339,10 +340,13 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> >   	if (pte_devmap(pte)) {
> >   		hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
> >   					      hmm_vma_walk->pgmap);
> > -		if (unlikely(!hmm_vma_walk->pgmap))
> > +		if (unlikely(!hmm_vma_walk->pgmap)) {
> > +			pte_unmap(ptep);
> >   			return -EBUSY;
> > +		}
> >   	} else if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pte_special(pte)) {
> >   		if (!is_zero_pfn(pte_pfn(pte))) {
> > +			pte_unmap(ptep);
> >   			*pfn = range->values[HMM_PFN_SPECIAL];
> >   			return -EFAULT;
> >   		}
> > @@ -437,7 +441,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
> >   		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
> >   		if (r) {
> > -			/* hmm_vma_handle_pte() did unmap pte directory */
> > +			/* hmm_vma_handle_pte() did pte_unmap() */
> >   			hmm_vma_walk->last = addr;
> >   			return r;
> >   		}
> > 
> 
> I think there is a case where hmm_vma_handle_pte() is called, a fault is requested,
> pte_unmap() and hmm_vma_walk_hole_() are called, the latter returns zero (the fault
> was handled OK)

Not quite, hmm_vma_walk_hole_() never returns 0 if called with fault:

        return (fault || write_fault) ? -EBUSY : 0;

And all the call sites of hmm_vma_walk_hole_() in hmm_vma_handle_pte()
are structured as:

                if (fault || write_fault)
                        goto fault;
fault:
        return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);

So, it never returns 0.

I already made a patch making this less twisty while fixing something
else:

https://github.com/jgunthorpe/linux/commit/078e10ca5919f2c263c245784fb5fe63ddbb61f4

Thanks,
Jason