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Message-ID: <49776f0a-d891-40be-bce3-c2b1f16bd825@nvidia.com>
Date: Mon, 15 Sep 2025 11:35:45 +1000
From: Balbir Singh <balbirs@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.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>,
Matthew Brost <matthew.brost@...el.com>,
Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v5 02/15] mm/huge_memory: add device-private THP support to PMD
operations
On 9/11/25 22:15, David Hildenbrand wrote:
> On 08.09.25 02:04, 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
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> 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>
>>
>> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>> ---
>> include/linux/swapops.h | 27 +++++++++++++++++++
>> mm/huge_memory.c | 60 ++++++++++++++++++++++++++++++++++++-----
>> mm/pgtable-generic.c | 6 +++++
>> 3 files changed, 86 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>> index 64ea151a7ae3..59c5889a4d54 100644
>> --- a/include/linux/swapops.h
>> +++ b/include/linux/swapops.h
>> @@ -594,6 +594,33 @@ 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;
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 26cedfcd7418..2af74e09b279 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1703,8 +1703,11 @@ 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)) {
>> + VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
>> + !is_pmd_device_private_entry(pmd));
>> +
>
> Wrong indentation.
checkpatch.pl did not complain, what I see is
+ VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
+ !is_pmd_device_private_entry(pmd));
It looks different in your reply (is your email converting tabs to space?
did you want me to align the conditions?
+ VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
+ !is_pmd_device_private_entry(pmd));
>
>> + if (is_migration_entry(entry) &&
>> + !is_readable_migration_entry(entry)) {
>
> Dito.
>
Same as above :)
> Wonder if we want to be more explicit.
>
> if (is_readable_migration_entry(enrty) ||
> is_readable_exclusive_migration_entry)) {
>
!is_readable_migration_entry => writable entry or read exclusive, did you mean is_writable_migration_entry()
above?
>
>> entry = make_readable_migration_entry(
>> swp_offset(entry));
>> pmd = swp_entry_to_pmd(entry);
>> @@ -1713,7 +1716,37 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>> 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));
>
> Put this on a single line.
>
Tried to keep it under 80 columns, but I see that checkpatch now allows upto 100 columns, will fix
>> + 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.
>> + */
>> + ret = folio_try_dup_anon_rmap_pmd(src_folio,
>> + &src_folio->page,
>> + dst_vma, src_vma);
>> + VM_WARN_ON(ret);
>
> Please just drop the ret + VM_WARN_ON here, like we did in the PTE case.
>
Ack
>> }
>> +
>> 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 +2244,17 @@ 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_migration_entry(orig_pmd) ||
>> + is_pmd_device_private_entry(orig_pmd)) {
>
>
> Indentation ...
>
Same as the discussion above
>> 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;
>> - } 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 +2274,12 @@ 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 +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_WARN_ON(!is_pmd_migration_entry(*pmd) &&
>> + !folio_is_device_private(folio));
>> if (is_writable_migration_entry(entry)) {
>> /*
>> * A protection check is difficult so
>> @@ -2380,6 +2422,10 @@ 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..604e8206a2ec 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -292,6 +292,12 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>> *pmdvalp = pmdval;
>> if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
>> goto nomap;
>> + if (is_swap_pmd(pmdval)) {
>> + swp_entry_t entry = pmd_to_swp_entry(pmdval);
>> +
>> + if (is_device_private_entry(entry))
>> + goto nomap;
>> + }
>
> Couldn't we do here
>
> if (!pmd_present(pmdval))
> goto nomap;
>
> To replace the original pmd_none() .. check.
>
> A page table must always be present IIRC.
>
I am not sure about the pmd_none(), a page table may not be present, I've not audited
the callers. But I think we can do
if (unlikely(pmd_none(pmdval)) || !pmd_present(pmdval))
goto nomap;
and remove the migration and device private entry checks
Balbir
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