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Message-ID: <CABzRoyYg1o8Oyjx1AQ8or-Vxm94zQXeAx7mWco2qs7=w4mBcMw@mail.gmail.com>
Date: Sun, 12 Oct 2025 23:46:39 +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 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?
> +
> 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;
> --
> 2.51.0
>
>
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