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Message-ID: <83492e9c-3f17-42e5-8897-9c0ed5aa76e7@linux.dev>
Date: Mon, 13 Oct 2025 09:48:35 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Balbir Singh <balbirs@...dia.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linux-mm@...ck.org, akpm@...ux-foundation.org,
David Hildenbrand <david@...hat.com>, 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: [v7 03/16] mm/huge_memory: add device-private THP support to PMD
operations
On 2025/10/13 08:01, Balbir Singh wrote:
> On 10/13/25 02:46, Lance Yang wrote:
>> On Wed, Oct 1, 2025 at 4:20 PM Balbir Singh <balbirs@...dia.com> 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: 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>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Acked-by: Zi Yan <ziy@...dia.com>
>>> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>>> Signed-off-by: Balbir Singh <balbirs@...dia.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);
>>> +}
>>> +
>>> #endif /* CONFIG_MMU */
>>> #endif /* _LINUX_SWAPOPS_H */
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 1b81680b4225..8e0a1747762d 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);
>>> }
>>> +
>>> 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));
>>> 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);
>>> + }
>>
>> IIUC, a device-private THP is always anonymous, right? would it make sense
>> to move this folio_is_device_private() block inside the folio_test_anon()
>> check above?
>>
> Yes, they are, there is discussion on file-backed mapping at
> https://lwn.net/Articles/1016124/. I don't see a benefit from moving it, do you?
Ah, I see. Never mind :)
Cheers,
Lance
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