[RFC 03/11] mm/thp: zone_device awareness in THP handling code
Balbir Singh
balbirs at nvidia.com
Wed Jul 9 12:30:42 UTC 2025
On 7/9/25 00:10, David Hildenbrand wrote:
> 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.
>
Ack
> 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".
>
The logic for the check is
if (pmd_present()) {
...
} else if (thp_migration_supported()) {
...
}
PMD swap is supported for migration entries (and zone device private after these changes)
>
>> +
>> + 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)
Are you suggesting refactoring of the code to add zap_nonpresent_pmd()? There really isn't much to be
done for specifically for migration entries in this context
>
>> } 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.
>
:)
Yep, this is wrong, we don't need to anything specific for anon_exclusive since
device private pages cannot be pinned. Other reviews have pointed towards fixes
needed as well.
>> + 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.
>
Ack, I'll try and use the presence of pmd to create the migration entries as appropriate
>> /* 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?
>
Fair point, we might not need the check here for THP handling.
>> 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.
>
The thp migration check is common to the pmd code checks even above the patched code, the
code checks for thp_migration() and PVMW_MIGRATION. If thp migration is not supported
is there any point in returning true?
> 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;
>
I don't think the code above is correct, you'll notice that there is a specific race
that the code handles for the !pmd_present() case and zap_huge_pmd() when PVMW_SYNC is set
I get the idea you're driving towards, I'll see how I can refactor it better.
> ...
> }
>
>> + 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(...))) {
>
> }
>
>
Will refactor to check for pmd_present first
Balbir
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