Re: [v2 11/16] mm: handle PMD swap entries in non-present PMD walkers

[Lance Yang]

On 2026/6/12 23:05, Usama Arif wrote:
> On 12/06/2026 07:45, Lance Yang wrote:
> > +Cc linux-mm
> >
> > Please Cc linux-mm next time. Pretty clearly MM work ...
>
> Yes, thanks for this! I forgot, will be careful in v3.

Cool.

> > On Tue, Jun 02, 2026 at 07:24:19AM -0700, Usama Arif wrote:
> > [...]
> > > diff --git a/mm/mincore.c b/mm/mincore.c
> > > index e5d13eea9234..3fee8a7b9d9d 100644
> > > --- a/mm/mincore.c
> > > +++ b/mm/mincore.c
> > > @@ -172,7 +172,19 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> > >
> > > 	ptl = pmd_trans_huge_lock(pmd, vma);
> > > 	if (ptl) {
> > > -		memset(vec, 1, nr);
> > > +		if (pmd_present(*pmd)) {
> > > +			memset(vec, 1, nr);
> > > +		} else {
> > > +			/*
> > > +			 * Non-present PMD: migration, device-private, or PMD
> > > +			 * swap entry. Route through mincore_swap() the same way
> > > +			 * the PTE path does -- the swap entry covers all 512
> > > +			 * slots, so the whole vec gets the same answer.
> > > +			 */
> > > +			softleaf_t entry = softleaf_from_pmd(*pmd);
> > > +
> > > +			memset(vec, mincore_swap(entry, false), nr);
> >
> > Looks buggy ...
> >
> > That assumes one swap-cache lookup is enough for whole PMD-sized range.
> > I don't think that always holds ...
> >
> > See do_huge_pmd_swap_page():
> >
> > ---8<---
> > 	folio = swap_cache_get_folio(swp_entry);
> > [...]
> > 	/*
> > 	 * Folio should be PMD-sized; if not (e.g. split in swap cache),
> > 	 * split the PMD swap entry and retry at PTE level.
> > 	 */
> > 	if (folio_nr_pages(folio) != HPAGE_PMD_NR) {
> > 		folio_unlock(folio);
> > 		folio_put(folio);
> > 		goto split_fallback;
> > 	}
> > ---
> >
> > it handles the case where swap_cache_get_folio() returns a folio that
> > is no longer PMD-sized. E.g. because it was split in the swap cache
> > while the PMD swap entry was installed. Then it split the PMD swap entry
> > and retries at PTE level :)
> >
> > unuse_pmd_entry() has the same fallback. Can mincore hit that case?
> >
> > Maybe the comment right above should say something like:
> >
> > "
> > One lookup is enough for a PMD-sized swapcache folio. If the swapcache
> > was split, check the per-page swap slots.
> > "
> >
> > Hopefully, I'm not missing something here :D
> >
> > Cheers, Lance
>
> Good catch! Thanks for pointing this out.
>
> I think the below diff over this commit should be ok. I will add
> it to the next revision. Its slower, but it shouldn't be an issue
> as its just mincore:

Just skimmed it. That should do the trick. Will go through it
properly in v3 :)

Thanks, Lance

> diff --git a/mm/mincore.c b/mm/mincore.c
> index 3fee8a7b9d9d..975513fff336 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -175,15 +175,42 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>                  if (pmd_present(*pmd)) {
>                          memset(vec, 1, nr);
>                  } else {
> -                       /*
> -                        * Non-present PMD: migration, device-private, or PMD
> -                        * swap entry. Route through mincore_swap() the same way
> -                        * the PTE path does -- the swap entry covers all 512
> -                        * slots, so the whole vec gets the same answer.
> -                        */
>                          softleaf_t entry = softleaf_from_pmd(*pmd);
>   
> -                       memset(vec, mincore_swap(entry, false), nr);
> +                       /*
> +                        * Non-present PMD: migration, device-private, or
> +                        * PMD swap entry. Migration / device-private cover
> +                        * the whole PMD range with a single answer.
> +                        */
> +                       if (!softleaf_is_swap(entry)) {
> +                               memset(vec, mincore_swap(entry, false), nr);
> +                       } else {
> +                               struct folio *folio = swap_cache_get_folio(entry);
> +
> +                               /*
> +                                * One lookup is enough for a PMD-sized
> +                                * swapcache folio. If the swapcache was split
> +                                * (e.g. by deferred_split_scan() or
> +                                * memory_failure()) while the PMD swap entry
> +                                * was installed, check the per-page swap slots.
> +                                */
> +                               if (folio && folio_nr_pages(folio) == HPAGE_PMD_NR) {
> +                                       memset(vec, folio_test_uptodate(folio), nr);
> +                                       folio_put(folio);
> +                               } else {
> +                                       unsigned long haddr = addr & HPAGE_PMD_MASK;
> +                                       pgoff_t off = swp_offset(entry) +
> +                                               ((addr - haddr) >> PAGE_SHIFT);
> +
> +                                       if (folio)
> +                                               folio_put(folio);
> +                                       for (i = 0; i < nr; i++)
> +                                               vec[i] = mincore_swap(
> +                                                       swp_entry(swp_type(entry),
> +                                                                 off + i),
> +                                                       false);
> +                               }
> +                       }
>                  }
>                  spin_unlock(ptl);
>                  goto out;