[RFC 03/11] mm/thp: zone_device awareness in THP handling code
David Hildenbrand
david at redhat.com
Tue Jul 8 14:10:55 UTC 2025
On 06.03.25 05:42, Balbir Singh wrote:
> Make THP handling code in the mm subsystem for THP pages
> aware of zone device pages. Although the code is
> designed to be generic when it comes to handling splitting
> of pages, the code is designed to work for THP page sizes
> corresponding to HPAGE_PMD_NR.
>
> Modify page_vma_mapped_walk() to return true when a zone
> device huge entry is present, enabling try_to_migrate()
> and other code migration paths to appropriately process the
> entry
>
> pmd_pfn() does not work well with zone device entries, use
> pfn_pmd_entry_to_swap() for checking and comparison as for
> zone device entries.
>
> try_to_map_to_unused_zeropage() does not apply to zone device
> entries, zone device entries are ignored in the call.
>
> Signed-off-by: Balbir Singh <balbirs at nvidia.com>
> ---
> mm/huge_memory.c | 151 +++++++++++++++++++++++++++++++------------
> mm/migrate.c | 2 +
> mm/page_vma_mapped.c | 10 +++
> mm/rmap.c | 19 +++++-
> 4 files changed, 138 insertions(+), 44 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 826bfe907017..d8e018d1bdbd 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2247,10 +2247,17 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> } else if (thp_migration_supported()) {
> swp_entry_t entry;
>
> - VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
> entry = pmd_to_swp_entry(orig_pmd);
> folio = pfn_swap_entry_folio(entry);
> flush_needed = 0;
> +
> + VM_BUG_ON(!is_pmd_migration_entry(*pmd) &&
> + !folio_is_device_private(folio));
Convert that to a VM_WARN_ON_ONCE() while you are at it.
But really, check that the *pmd* is as expected (device_pritavte entry),
and not the folio after the effects.
Also, hiding all that under the thp_migration_supported() looks wrong.
Likely you must clean that up first, to have something that expresses
that we support PMD swap entries or sth like that. Not just "migration
entries".
> +
> + if (folio_is_device_private(folio)) {
> + folio_remove_rmap_pmd(folio, folio_page(folio, 0), vma);
> + WARN_ON_ONCE(folio_mapcount(folio) < 0);
> + }
zap_nonpresent_ptes() does
if (is_device_private_entry(entry)) {
...
} else if (is_migration_entry(entry)) {
....
}
Can we adjust the same way of foing things? (yes, we might want a
thp_migration_supported() check somewhere)
> } else
> WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
>
> @@ -2264,6 +2271,15 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> -HPAGE_PMD_NR);
> }
>
> + /*
> + * Do a folio put on zone device private pages after
> + * changes to mm_counter, because the folio_put() will
> + * clean folio->mapping and the folio_test_anon() check
> + * will not be usable.
> + */
> + if (folio_is_device_private(folio))
> + folio_put(folio);
> +
> spin_unlock(ptl);
> if (flush_needed)
> tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
> @@ -2392,7 +2408,8 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> struct folio *folio = pfn_swap_entry_folio(entry);
> pmd_t newpmd;
>
> - VM_BUG_ON(!is_pmd_migration_entry(*pmd));
> + VM_BUG_ON(!is_pmd_migration_entry(*pmd) &&
> + !folio_is_device_private(folio));
> if (is_writable_migration_entry(entry)) {
> /*
> * A protection check is difficult so
> @@ -2405,9 +2422,11 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> newpmd = swp_entry_to_pmd(entry);
> if (pmd_swp_soft_dirty(*pmd))
> newpmd = pmd_swp_mksoft_dirty(newpmd);
> - } else {
> + } else if (is_writable_device_private_entry(entry)) {
> + newpmd = swp_entry_to_pmd(entry);
> + entry = make_device_exclusive_entry(swp_offset(entry));
> + } else
> newpmd = *pmd;
> - }
>
> if (uffd_wp)
> newpmd = pmd_swp_mkuffd_wp(newpmd);
> @@ -2860,11 +2879,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> struct page *page;
> pgtable_t pgtable;
> pmd_t old_pmd, _pmd;
> - bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
> - bool anon_exclusive = false, dirty = false;
> + bool young, write, soft_dirty, uffd_wp = false;
> + bool anon_exclusive = false, dirty = false, present = false;
> unsigned long addr;
> pte_t *pte;
> int i;
> + swp_entry_t swp_entry;
>
> VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
> VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
> @@ -2918,20 +2938,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> return __split_huge_zero_page_pmd(vma, haddr, pmd);
> }
>
> - pmd_migration = is_pmd_migration_entry(*pmd);
> - if (unlikely(pmd_migration)) {
> - swp_entry_t entry;
>
> + present = pmd_present(*pmd);
> + if (unlikely(!present)) {
> + swp_entry = pmd_to_swp_entry(*pmd);
> old_pmd = *pmd;
> - entry = pmd_to_swp_entry(old_pmd);
> - page = pfn_swap_entry_to_page(entry);
> - write = is_writable_migration_entry(entry);
> +
> + folio = pfn_swap_entry_folio(swp_entry);
> + VM_BUG_ON(!is_migration_entry(swp_entry) &&
> + !is_device_private_entry(swp_entry));
> + page = pfn_swap_entry_to_page(swp_entry);
> + write = is_writable_migration_entry(swp_entry);
> +
> if (PageAnon(page))
> - anon_exclusive = is_readable_exclusive_migration_entry(entry);
> - young = is_migration_entry_young(entry);
> - dirty = is_migration_entry_dirty(entry);
> + anon_exclusive =
> + is_readable_exclusive_migration_entry(swp_entry);
> soft_dirty = pmd_swp_soft_dirty(old_pmd);
> uffd_wp = pmd_swp_uffd_wp(old_pmd);
> + young = is_migration_entry_young(swp_entry);
> + dirty = is_migration_entry_dirty(swp_entry);
> } else {
> /*
> * Up to this point the pmd is present and huge and userland has
> @@ -3015,30 +3040,45 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> * Note that NUMA hinting access restrictions are not transferred to
> * avoid any possibility of altering permissions across VMAs.
> */
> - if (freeze || pmd_migration) {
> + if (freeze || !present) {
> for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> pte_t entry;
> - swp_entry_t swp_entry;
> -
> - if (write)
> - swp_entry = make_writable_migration_entry(
> - page_to_pfn(page + i));
> - else if (anon_exclusive)
> - swp_entry = make_readable_exclusive_migration_entry(
> - page_to_pfn(page + i));
> - else
> - swp_entry = make_readable_migration_entry(
> - page_to_pfn(page + i));
> - if (young)
> - swp_entry = make_migration_entry_young(swp_entry);
> - if (dirty)
> - swp_entry = make_migration_entry_dirty(swp_entry);
> - entry = swp_entry_to_pte(swp_entry);
> - if (soft_dirty)
> - entry = pte_swp_mksoft_dirty(entry);
> - if (uffd_wp)
> - entry = pte_swp_mkuffd_wp(entry);
> -
> + if (freeze || is_migration_entry(swp_entry)) {
> + if (write)
> + swp_entry = make_writable_migration_entry(
> + page_to_pfn(page + i));
> + else if (anon_exclusive)
> + swp_entry = make_readable_exclusive_migration_entry(
> + page_to_pfn(page + i));
> + else
> + swp_entry = make_readable_migration_entry(
> + page_to_pfn(page + i));
> + if (young)
> + swp_entry = make_migration_entry_young(swp_entry);
> + if (dirty)
> + swp_entry = make_migration_entry_dirty(swp_entry);
> + entry = swp_entry_to_pte(swp_entry);
> + if (soft_dirty)
> + entry = pte_swp_mksoft_dirty(entry);
> + if (uffd_wp)
> + entry = pte_swp_mkuffd_wp(entry);
> + } else {
> + VM_BUG_ON(!is_device_private_entry(swp_entry));
> + if (write)
> + swp_entry = make_writable_device_private_entry(
> + page_to_pfn(page + i));
> + else if (anon_exclusive)
> + swp_entry = make_device_exclusive_entry(
> + page_to_pfn(page + i));
I am pretty sure this is wrong. You cannot suddenly mix in
device-exclusive entries.
And now I am confused again how device-private, anon and GUP interact.
> + else
> + swp_entry = make_readable_device_private_entry(
> + page_to_pfn(page + i));
> + entry = swp_entry_to_pte(swp_entry);
> + if (soft_dirty)
> + entry = pte_swp_mksoft_dirty(entry);
> + if (uffd_wp)
> + entry = pte_swp_mkuffd_wp(entry);
> + }
> VM_WARN_ON(!pte_none(ptep_get(pte + i)));
> set_pte_at(mm, addr, pte + i, entry);
> }
> @@ -3065,7 +3105,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> }
> pte_unmap(pte);
>
> - if (!pmd_migration)
> + if (present)
> folio_remove_rmap_pmd(folio, page, vma);
> if (freeze)
> put_page(page);
> @@ -3077,6 +3117,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> pmd_t *pmd, bool freeze, struct folio *folio)
> {
> + struct folio *pmd_folio;
> VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
> VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
> VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> @@ -3089,7 +3130,14 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> */
> if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> is_pmd_migration_entry(*pmd)) {
> - if (folio && folio != pmd_folio(*pmd))
> + if (folio && !pmd_present(*pmd)) {
> + swp_entry_t swp_entry = pmd_to_swp_entry(*pmd);
> +
> + pmd_folio = page_folio(pfn_swap_entry_to_page(swp_entry));
> + } else {
> + pmd_folio = pmd_folio(*pmd);
> + }
> + if (folio && folio != pmd_folio)
> return;
> __split_huge_pmd_locked(vma, pmd, address, freeze);
> }
> @@ -3581,11 +3629,16 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
> folio_test_swapcache(origin_folio)) ?
> folio_nr_pages(release) : 0));
>
> + if (folio_is_device_private(release))
> + percpu_ref_get_many(&release->pgmap->ref,
> + (1 << new_order) - 1);
> +
> if (release == origin_folio)
> continue;
>
> - lru_add_page_tail(origin_folio, &release->page,
> - lruvec, list);
> + if (!folio_is_device_private(origin_folio))
> + lru_add_page_tail(origin_folio, &release->page,
> + lruvec, list);
>
> /* Some pages can be beyond EOF: drop them from page cache */
> if (release->index >= end) {
> @@ -4625,7 +4678,10 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
> return 0;
>
> flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
> - pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
> + if (!folio_is_device_private(folio))
> + pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
> + else
> + pmdval = pmdp_huge_clear_flush(vma, address, pvmw->pmd);
Please handle this like we handle the PTE case -- checking for
pmd_present() instead.
Avoid placing these nasty folio_is_device_private() all over the place
where avoidable.
>
> /* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */
> anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(page);
> @@ -4675,6 +4731,17 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
> entry = pmd_to_swp_entry(*pvmw->pmd);
> folio_get(folio);
> pmde = mk_huge_pmd(new, READ_ONCE(vma->vm_page_prot));
> +
> + if (unlikely(folio_is_device_private(folio))) {
> + if (pmd_write(pmde))
> + entry = make_writable_device_private_entry(
> + page_to_pfn(new));
> + else
> + entry = make_readable_device_private_entry(
> + page_to_pfn(new));
> + pmde = swp_entry_to_pmd(entry);
> + }
> +
> if (pmd_swp_soft_dirty(*pvmw->pmd))
> pmde = pmd_mksoft_dirty(pmde);
> if (is_writable_migration_entry(entry))
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 59e39aaa74e7..0aa1bdb711c3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -200,6 +200,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>
> if (PageCompound(page))
> return false;
> + if (folio_is_device_private(folio))
> + return false;
Why is that check required when you are adding THP handling and there is
a PageCompound check right there?
> VM_BUG_ON_PAGE(!PageAnon(page), page);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index e463c3be934a..5dd2e51477d3 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -278,6 +278,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> * cannot return prematurely, while zap_huge_pmd() has
> * cleared *pmd but not decremented compound_mapcount().
> */
> + swp_entry_t entry;
> +
> + if (!thp_migration_supported())
> + return not_found(pvmw);
This check looks misplaced. We should follow the same model as check_pte().
Checking for THP migration support when you are actually caring about
device-private entries is weird.
That is, I would expect something like
} else if (is_swap_pmd(pmde)) {
swp_entry_t entry;
entry = pmd_to_swp_entry(pmde);
if (!is_device_private_entry(entry))
return false;
...
}
> + entry = pmd_to_swp_entry(pmde);
> + if (is_device_private_entry(entry)) {
> + pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> + return true;
> + }
> +
> if ((pvmw->flags & PVMW_SYNC) &&
> thp_vma_suitable_order(vma, pvmw->address,
> PMD_ORDER) &&
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 67bb273dfb80..67e99dc5f2ef 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2326,8 +2326,23 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> /* PMD-mapped THP migration entry */
> if (!pvmw.pte) {
> - subpage = folio_page(folio,
> - pmd_pfn(*pvmw.pmd) - folio_pfn(folio));
> + /*
> + * Zone device private folios do not work well with
> + * pmd_pfn() on some architectures due to pte
> + * inversion.
> + */
> + if (folio_is_device_private(folio)) {
> + swp_entry_t entry = pmd_to_swp_entry(*pvmw.pmd);
> + unsigned long pfn = swp_offset_pfn(entry);
> +
> + subpage = folio_page(folio, pfn
> + - folio_pfn(folio));
> + } else {
> + subpage = folio_page(folio,
> + pmd_pfn(*pvmw.pmd)
> + - folio_pfn(folio));
> + }
> +
Please follow the same model we use for PTEs.
/*
* Handle PFN swap PMDs, such as device-exclusive ones, that
* actually map pages.
*/
if (likely(pmd_present(...))) {
}
--
Cheers,
David / dhildenb
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