On Wed, Mar 11, 2020 at 06:36:47PM -0700, Ralph Campbell wrote:
> > @@ -390,8 +384,15 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
> > return -EBUSY;
> > }
> > return hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
> > - } else if (!pmd_present(pmd))
> > + }
> > +
> > + if (!pmd_present(pmd)) {
> > + hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
> > + &write_fault);
> > + if (fault || write_fault)
> > + return -EFAULT;
> > return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
>
> Shouldn't this fill with HMM_PFN_NONE instead of HMM_PFN_ERROR?
> Otherwise, when a THP is swapped out, you will get a different
> value than if a PTE is swapped out and you are prefetching/snapshotting.
If this is the case then the problem is that the return -EFAULT path
needs to do something else.. ie since the above code can't trigger
swap in, it is correct to return PFN_ERROR.
I'm completely guessing, but do we need to call pmd_to_swp_entry() and
handle things similarly to the pte? What swp_entries are valid for a
pmd?
Do you understand this better, or know how to trigger a !pmd_present
for test?
I suppose another option would be this:
if (!pmd_present(pmd)) {
hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
&write_fault);
/* We can't handle this. Cause the PMD to be split and
* handle it in the pte handler. */
if (fault || write_fault)
return 0;
return hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
}
Which, I think, must be correct, but inefficient?
Jason
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