Re: [v1 resend 00/12] THP support for zone device page migration

[Francois Dugast <francois.dugast@intel.com>]

On Fri, Jul 18, 2025 at 01:57:13PM +1000, Balbir Singh wrote:
> On 7/18/25 09:40, Matthew Brost wrote:
> > On Fri, Jul 04, 2025 at 09:34:59AM +1000, Balbir Singh wrote:
> ...
> >>
> >> The nouveau dmem code has been enhanced to use the new THP migration
> >> capability.
> >>
> >> Feedback from the RFC [2]:
> >>
> > 
> > Thanks for the patches, results look very promising. I wanted to give
> > some quick feedback:
> > 
> 
> Are you seeing improvements with the patchset?
> 
> > - You appear to have missed updating hmm_range_fault, specifically
> > hmm_vma_handle_pmd, to check for device-private entries and populate the
> > HMM PFNs accordingly. My colleague François has a fix for this if you're
> > interested.
> > 
> 
> Sure, please feel free to post them. 

Hi Balbir,

It seems we are missing this special handling in in hmm_vma_walk_pmd():

diff --git a/mm/hmm.c b/mm/hmm.c
index f2415b4b2cdd..449025f72b2f 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -355,6 +355,27 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
        }
 
        if (!pmd_present(pmd)) {
+               swp_entry_t entry = pmd_to_swp_entry(pmd);
+
+               /*
+                * Don't fault in device private pages owned by the caller,
+                * just report the PFNs.
+                */
+               if (is_device_private_entry(entry) &&
+                   pfn_swap_entry_folio(entry)->pgmap->owner ==
+                   range->dev_private_owner) {
+                       unsigned long cpu_flags = pmd_to_hmm_pfn_flags(range, pmd);
+                       unsigned long pfn = swp_offset_pfn(entry);
+                       unsigned long i;
+
+                       for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
+                               hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
+                               hmm_pfns[i] |= pfn | cpu_flags;
+                       }
+
+                       return 0;
+               }
+
                if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
                        return -EFAULT;
                return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);

Francois

> 
> > - I believe copy_huge_pmd also needs to be updated to avoid installing a
> > migration entry if the swap entry is device-private. I don't have an
> > exact fix yet due to my limited experience with core MM. The test case
> > that triggers this is fairly simple: fault in a 2MB device page on the
> > GPU, then fork a process that reads the page — the kernel crashes in
> > this scenario.
> > 
> 
> I'd be happy to look at any traces you have or post any fixes you have
> 
> Thanks for the feedback
> Balbir Singh
>