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Message-ID: <5b9da42e-49a2-4229-81ce-625ce00ed7b2@nvidia.com>
Date: Fri, 19 Sep 2025 14:51:20 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, damon@...ts.linux.dev,
dri-devel@...ts.freedesktop.org, Matthew Brost <matthew.brost@...el.com>,
David Hildenbrand <david@...hat.com>, Joshua Hahn <joshua.hahnjy@...il.com>,
Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
Gregory Price <gourry@...rry.net>, Ying Huang
<ying.huang@...ux.alibaba.com>, Alistair Popple <apopple@...dia.com>,
Oscar Salvador <osalvador@...e.de>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
Mika Penttilä <mpenttil@...hat.com>,
Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v6 02/15] mm/huge_memory: add device-private THP support to PMD
operations
On 9/19/25 04:45, Zi Yan wrote:
> On 16 Sep 2025, at 8:21, Balbir Singh wrote:
>
>> Extend core huge page management functions to handle device-private THP
>> entries. This enables proper handling of large device-private folios in
>> fundamental MM operations.
>>
>> The following functions have been updated:
>>
>> - copy_huge_pmd(): Handle device-private entries during fork/clone
>> - zap_huge_pmd(): Properly free device-private THP during munmap
>> - change_huge_pmd(): Support protection changes on device-private THP
>> - __pte_offset_map(): Add device-private entry awareness
>>
>> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Zi Yan <ziy@...dia.com>
>> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
>> Cc: Rakie Kim <rakie.kim@...com>
>> Cc: Byungchul Park <byungchul@...com>
>> Cc: Gregory Price <gourry@...rry.net>
>> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>> Cc: Nico Pache <npache@...hat.com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Dev Jain <dev.jain@....com>
>> Cc: Barry Song <baohua@...nel.org>
>> Cc: Lyude Paul <lyude@...hat.com>
>> Cc: Danilo Krummrich <dakr@...nel.org>
>> Cc: David Airlie <airlied@...il.com>
>> Cc: Simona Vetter <simona@...ll.ch>
>> Cc: Ralph Campbell <rcampbell@...dia.com>
>> Cc: Mika Penttilä <mpenttil@...hat.com>
>> Cc: Matthew Brost <matthew.brost@...el.com>
>> Cc: Francois Dugast <francois.dugast@...el.com>
>> ---
>> include/linux/swapops.h | 32 +++++++++++++++++++++++
>> mm/huge_memory.c | 56 ++++++++++++++++++++++++++++++++++-------
>> mm/pgtable-generic.c | 2 +-
>> 3 files changed, 80 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>> index 64ea151a7ae3..2687928a8146 100644
>> --- a/include/linux/swapops.h
>> +++ b/include/linux/swapops.h
>> @@ -594,10 +594,42 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
>> }
>> #endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
>>
>> +#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_ARCH_ENABLE_THP_MIGRATION)
>> +
>> +/**
>> + * is_pmd_device_private_entry() - Check if PMD contains a device private swap entry
>> + * @pmd: The PMD to check
>> + *
>> + * Returns true if the PMD contains a swap entry that represents a device private
>> + * page mapping. This is used for zone device private pages that have been
>> + * swapped out but still need special handling during various memory management
>> + * operations.
>> + *
>> + * Return: 1 if PMD contains device private entry, 0 otherwise
>> + */
>> +static inline int is_pmd_device_private_entry(pmd_t pmd)
>> +{
>> + return is_swap_pmd(pmd) && is_device_private_entry(pmd_to_swp_entry(pmd));
>> +}
>> +
>> +#else /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
>> +
>> +static inline int is_pmd_device_private_entry(pmd_t pmd)
>> +{
>> + return 0;
>> +}
>> +
>> +#endif /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
>> +
>> static inline int non_swap_entry(swp_entry_t entry)
>> {
>> return swp_type(entry) >= MAX_SWAPFILES;
>> }
>>
>> +static inline int is_pmd_non_present_folio_entry(pmd_t pmd)
>> +{
>> + return is_pmd_migration_entry(pmd) || is_pmd_device_private_entry(pmd);
>> +}
>> +
>
> non_present seems too vague. Maybe just open code it.
This was David's suggestion from the previous posting, there is is_swap_pfn_entry()
but it's much larger than we would like for our use case.
>
>
>> #endif /* CONFIG_MMU */
>> #endif /* _LINUX_SWAPOPS_H */
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 5acca24bbabb..a5e4c2aef191 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1703,17 +1703,45 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>> if (unlikely(is_swap_pmd(pmd))) {
>> swp_entry_t entry = pmd_to_swp_entry(pmd);
>>
>> - VM_BUG_ON(!is_pmd_migration_entry(pmd));
>> - if (!is_readable_migration_entry(entry)) {
>> - entry = make_readable_migration_entry(
>> - swp_offset(entry));
>> + VM_WARN_ON(!is_pmd_non_present_folio_entry(pmd));
>> +
>> + if (is_writable_migration_entry(entry) ||
>> + is_readable_exclusive_migration_entry(entry)) {
>> + entry = make_readable_migration_entry(swp_offset(entry));
>> pmd = swp_entry_to_pmd(entry);
>> if (pmd_swp_soft_dirty(*src_pmd))
>> pmd = pmd_swp_mksoft_dirty(pmd);
>> if (pmd_swp_uffd_wp(*src_pmd))
>> pmd = pmd_swp_mkuffd_wp(pmd);
>> set_pmd_at(src_mm, addr, src_pmd, pmd);
>> + } else if (is_device_private_entry(entry)) {
>> + /*
>> + * For device private entries, since there are no
>> + * read exclusive entries, writable = !readable
>> + */
>> + if (is_writable_device_private_entry(entry)) {
>> + entry = make_readable_device_private_entry(swp_offset(entry));
>> + pmd = swp_entry_to_pmd(entry);
>> +
>> + if (pmd_swp_soft_dirty(*src_pmd))
>> + pmd = pmd_swp_mksoft_dirty(pmd);
>> + if (pmd_swp_uffd_wp(*src_pmd))
>> + pmd = pmd_swp_mkuffd_wp(pmd);
>> + set_pmd_at(src_mm, addr, src_pmd, pmd);
>> + }
>> +
>> + src_folio = pfn_swap_entry_folio(entry);
>> + VM_WARN_ON(!folio_test_large(src_folio));
>> +
>> + folio_get(src_folio);
>> + /*
>> + * folio_try_dup_anon_rmap_pmd does not fail for
>> + * device private entries.
>> + */
>> + folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page,
>> + dst_vma, src_vma);’
>
> folio_get() and folio_try_dup_anon_rmap_pmd() are needed, because
> contrary to the migration entry case, this folio exists as
> a device private one.
>
Is that a question?
>> }
>> +
>> add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>> mm_inc_nr_ptes(dst_mm);
>> pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
>> @@ -2211,15 +2239,16 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>> folio_remove_rmap_pmd(folio, page, vma);
>> WARN_ON_ONCE(folio_mapcount(folio) < 0);
>> VM_BUG_ON_PAGE(!PageHead(page), page);
>> - } else if (thp_migration_supported()) {
>> + } else if (is_pmd_non_present_folio_entry(orig_pmd)) {
>> swp_entry_t entry;
>>
>> - VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
>
> It implies thp_migration_supported() is true here. We could have
> VM_WARN_ONCE_ON(!thp_migration_supported()), but that might be too much.
>
Yes, since we've validated that this is a pmd migration or device
private entry.
>> entry = pmd_to_swp_entry(orig_pmd);
>> folio = pfn_swap_entry_folio(entry);
>> flush_needed = 0;
>> - } else
>> - WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
>> +
>> + if (!thp_migration_supported())
>> + WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
>> + }
>>
>> if (folio_test_anon(folio)) {
>> zap_deposited_table(tlb->mm, pmd);
>> @@ -2239,6 +2268,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>> folio_mark_accessed(folio);
>> }
>>
>> + if (folio_is_device_private(folio)) {
>> + folio_remove_rmap_pmd(folio, &folio->page, vma);
>> + WARN_ON_ONCE(folio_mapcount(folio) < 0);
>> + folio_put(folio);
>> + }
>> +
>> spin_unlock(ptl);
>> if (flush_needed)
>> tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
>> @@ -2367,7 +2402,7 @@ 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_WARN_ON(!is_pmd_non_present_folio_entry(*pmd));
>> if (is_writable_migration_entry(entry)) {
>> /*
>> * A protection check is difficult so
>> @@ -2380,6 +2415,9 @@ 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 if (is_writable_device_private_entry(entry)) {
>> + entry = make_readable_device_private_entry(swp_offset(entry));
>> + newpmd = swp_entry_to_pmd(entry);
>> } else {
>> newpmd = *pmd;
>> }
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 567e2d084071..0c847cdf4fd3 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -290,7 +290,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>>
>> if (pmdvalp)
>> *pmdvalp = pmdval;
>> - if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
>> + if (unlikely(pmd_none(pmdval) || !pmd_present(pmdval)))
>> goto nomap;
>> if (unlikely(pmd_trans_huge(pmdval)))
>> goto nomap;
>> --
>> 2.50.1
>
> Otherwise, LGTM. Acked-by: Zi Yan <ziy@...dia.com>
>
Thanks Zi!
Balbir
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