Re: [RFC PATCH 4/6] mm: Add a new swap type for migration entries with device private PFNs

Chih-En Lin
2025-12-01T02:43:46Z

On Fri, Nov 28, 2025 at 03:41:44PM +1100, Jordan Niethe wrote:
> A future change will remove device private pages from the physical
> address space. This will mean that device private pages no longer have
> normal PFN and must be handled separately.
>
> When migrating a device private page a migration entry is created for
> that page - this includes the PFN for that page. Once device private
> PFNs exist in a different address space to regular PFNs we need to be
> able to determine which kind of PFN is in the entry so we can associate
> it with the correct page.
>
> Introduce new swap types:
>
> - SWP_MIGRATION_DEVICE_READ
> - SWP_MIGRATION_DEVICE_WRITE
> - SWP_MIGRATION_DEVICE_READ_EXCLUSIVE
>
> These correspond to
>
> - SWP_MIGRATION_READ
> - SWP_MIGRATION_WRITE
> - SWP_MIGRATION_READ_EXCLUSIVE
>
> except the swap entry contains a device private PFN.
>
> The existing helpers such as is_writable_migration_entry() will still
> return true for a SWP_MIGRATION_DEVICE_WRITE entry.
>
> Introduce new helpers such as
> is_writable_device_migration_private_entry() to disambiguate between a
> SWP_MIGRATION_WRITE and a SWP_MIGRATION_DEVICE_WRITE entry.
>
> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> include/linux/swap.h | 8 +++-
> include/linux/swapops.h | 87 ++++++++++++++++++++++++++++++++++++++---
> mm/memory.c | 9 ++++-
> mm/migrate.c | 2 +-
> mm/migrate_device.c | 31 ++++++++++-----
> mm/mprotect.c | 21 +++++++---
> mm/page_vma_mapped.c | 2 +-
> mm/pagewalk.c | 3 +-
> mm/rmap.c | 32 ++++++++++-----
> 9 files changed, 161 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index e818fbade1e2..87f14d673979 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -74,12 +74,18 @@ static inline int current_is_kswapd(void)
> *
> * When a page is mapped by the device for exclusive access we set the CPU page
> * table entries to a special SWP_DEVICE_EXCLUSIVE entry.
> + *
> + * Because device private pages do not use regular PFNs, special migration
> + * entries are also needed.
> */
> #ifdef CONFIG_DEVICE_PRIVATE
> -#define SWP_DEVICE_NUM 3
> +#define SWP_DEVICE_NUM 6
> #define SWP_DEVICE_WRITE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM)
> #define SWP_DEVICE_READ (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+1)
> #define SWP_DEVICE_EXCLUSIVE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+2)
> +#define SWP_MIGRATION_DEVICE_READ (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+3)
> +#define SWP_MIGRATION_DEVICE_READ_EXCLUSIVE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+4)
> +#define SWP_MIGRATION_DEVICE_WRITE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+5)
> #else
> #define SWP_DEVICE_NUM 0
> #endif
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 64ea151a7ae3..7aa3f00e304a 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -196,6 +196,43 @@ static inline bool is_device_exclusive_entry(swp_entry_t entry)
> return swp_type(entry) == SWP_DEVICE_EXCLUSIVE;
> }
>
> +static inline swp_entry_t make_readable_migration_device_private_entry(pgoff_t offset)
> +{
> + return swp_entry(SWP_MIGRATION_DEVICE_READ, offset);
> +}
> +
> +static inline swp_entry_t make_writable_migration_device_private_entry(pgoff_t offset)
> +{
> + return swp_entry(SWP_MIGRATION_DEVICE_WRITE, offset);
> +}
> +
> +static inline bool is_device_private_migration_entry(swp_entry_t entry)
> +{
> + return unlikely(swp_type(entry) == SWP_MIGRATION_DEVICE_READ ||
> + swp_type(entry) == SWP_MIGRATION_DEVICE_READ_EXCLUSIVE ||
> + swp_type(entry) == SWP_MIGRATION_DEVICE_WRITE);
> +}
> +
> +static inline bool is_readable_device_migration_private_entry(swp_entry_t entry)
> +{
> + return unlikely(swp_type(entry) == SWP_MIGRATION_DEVICE_READ);
> +}
> +
> +static inline bool is_writable_device_migration_private_entry(swp_entry_t entry)
> +{
> + return unlikely(swp_type(entry) == SWP_MIGRATION_DEVICE_WRITE);
> +}
> +
> +static inline swp_entry_t make_device_migration_readable_exclusive_migration_entry(pgoff_t offset)
> +{
> + return swp_entry(SWP_MIGRATION_DEVICE_READ_EXCLUSIVE, offset);
> +}
> +
> +static inline bool is_device_migration_readable_exclusive_entry(swp_entry_t entry)
> +{
> + return swp_type(entry) == SWP_MIGRATION_DEVICE_READ_EXCLUSIVE;
> +}

The names are inconsistent.

Maybe make_device_migration_readable_exclusive_migration_entry to
make_readable_exclusive_migration_device_private_entry, and
is_device_migration_readable_exclusive_entry to
is_readable_exclusive_device_private_migration_entry?

> #else /* CONFIG_DEVICE_PRIVATE */
> static inline swp_entry_t make_readable_device_private_entry(pgoff_t offset)
> {
> @@ -217,6 +254,11 @@ static inline bool is_writable_device_private_entry(swp_entry_t entry)
> return false;
> }
>
> +static inline bool is_readable_device_migration_private_entry(swp_entry_t entry)
> +{
> + return false;
> +}
> +
> static inline swp_entry_t make_device_exclusive_entry(pgoff_t offset)
> {
> return swp_entry(0, 0);
> @@ -227,6 +269,36 @@ static inline bool is_device_exclusive_entry(swp_entry_t entry)
> return false;
> }
>
> +static inline swp_entry_t make_readable_migration_device_private_entry(pgoff_t offset)
> +{
> + return swp_entry(0, 0);
> +}
> +
> +static inline swp_entry_t make_writable_migration_device_private_entry(pgoff_t offset)
> +{
> + return swp_entry(0, 0);
> +}
> +
> +static inline bool is_device_private_migration_entry(swp_entry_t entry)
> +{
> + return false;
> +}
> +
> +static inline bool is_writable_device_migration_private_entry(swp_entry_t entry)
> +{
> + return false;
> +}
> +
> +static inline swp_entry_t make_device_migration_readable_exclusive_migration_entry(pgoff_t offset)
> +{
> + return swp_entry(0, 0);
> +}
> +
> +static inline bool is_device_migration_readable_exclusive_entry(swp_entry_t entry)
> +{
> + return false;
> +}
> +
> #endif /* CONFIG_DEVICE_PRIVATE */
>
> #ifdef CONFIG_MIGRATION
> @@ -234,22 +306,26 @@ static inline int is_migration_entry(swp_entry_t entry)
> {
> return unlikely(swp_type(entry) == SWP_MIGRATION_READ ||
> swp_type(entry) == SWP_MIGRATION_READ_EXCLUSIVE ||
> - swp_type(entry) == SWP_MIGRATION_WRITE);
> + swp_type(entry) == SWP_MIGRATION_WRITE ||
> + is_device_private_migration_entry(entry));
> }
>
> static inline int is_writable_migration_entry(swp_entry_t entry)
> {
> - return unlikely(swp_type(entry) == SWP_MIGRATION_WRITE);
> + return unlikely(swp_type(entry) == SWP_MIGRATION_WRITE ||
> + is_writable_device_migration_private_entry(entry));
> }
>
> static inline int is_readable_migration_entry(swp_entry_t entry)
> {
> - return unlikely(swp_type(entry) == SWP_MIGRATION_READ);
> + return unlikely(swp_type(entry) == SWP_MIGRATION_READ ||
> + is_readable_device_migration_private_entry(entry));
> }
>
> static inline int is_readable_exclusive_migration_entry(swp_entry_t entry)
> {
> - return unlikely(swp_type(entry) == SWP_MIGRATION_READ_EXCLUSIVE);
> + return unlikely(swp_type(entry) == SWP_MIGRATION_READ_EXCLUSIVE ||
> + is_device_migration_readable_exclusive_entry(entry));
> }
>
> static inline swp_entry_t make_readable_migration_entry(pgoff_t offset)
> @@ -525,7 +601,8 @@ static inline bool is_pfn_swap_entry(swp_entry_t entry)
> BUILD_BUG_ON(SWP_TYPE_SHIFT < SWP_PFN_BITS);
>
> return is_migration_entry(entry) || is_device_private_entry(entry) ||
> - is_device_exclusive_entry(entry) || is_hwpoison_entry(entry);
> + is_device_exclusive_entry(entry) || is_hwpoison_entry(entry) ||
> + is_device_private_migration_entry(entry);
> }
>
> struct page_vma_mapped_walk;
> diff --git a/mm/memory.c b/mm/memory.c
> index b59ae7ce42eb..f1ed361434ff 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -962,8 +962,13 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> * to be set to read. A previously exclusive entry is
> * now shared.
> */
> - entry = make_readable_migration_entry(
> - swp_offset(entry));
> + if (is_device_private_migration_entry(entry))
> + entry = make_readable_migration_device_private_entry(
> + swp_offset(entry));
> + else
> + entry = make_readable_migration_entry(
> + swp_offset(entry));
> +
> pte = swp_entry_to_pte(entry);
> if (pte_swp_soft_dirty(orig_pte))
> pte = pte_swp_mksoft_dirty(pte);
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c0e9f15be2a2..3c561d61afba 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -495,7 +495,7 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
> goto out;
>
> entry = pte_to_swp_entry(pte);
> - if (!is_migration_entry(entry))
> + if (!(is_migration_entry(entry)))
> goto out;
>
> migration_entry_wait_on_locked(entry, ptl);
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 82f09b24d913..458b5114bb2b 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -235,15 +235,28 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> folio_mark_dirty(folio);
>
> /* Setup special migration page table entry */
> - if (mpfn & MIGRATE_PFN_WRITE)
> - entry = make_writable_migration_entry(
> - page_to_pfn(page));
> - else if (anon_exclusive)
> - entry = make_readable_exclusive_migration_entry(
> - page_to_pfn(page));
> - else
> - entry = make_readable_migration_entry(
> - page_to_pfn(page));
> + if (mpfn & MIGRATE_PFN_WRITE) {
> + if (is_device_private_page(page))
> + entry = make_writable_migration_device_private_entry(
> + page_to_pfn(page));
> + else
> + entry = make_writable_migration_entry(
> + page_to_pfn(page));
> + } else if (anon_exclusive) {
> + if (is_device_private_page(page))
> + entry = make_device_migration_readable_exclusive_migration_entry(
> + page_to_pfn(page));
> + else
> + entry = make_readable_exclusive_migration_entry(
> + page_to_pfn(page));
> + } else {
> + if (is_device_private_page(page))
> + entry = make_readable_migration_device_private_entry(
> + page_to_pfn(page));
> + else
> + entry = make_readable_migration_entry(
> + page_to_pfn(page));
> + }
> if (pte_present(pte)) {
> if (pte_young(pte))
> entry = make_migration_entry_young(entry);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 113b48985834..7d79a0f53bf5 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -365,11 +365,22 @@ static long change_pte_range(struct mmu_gather *tlb,
> * A protection check is difficult so
> * just be safe and disable write
> */
> - if (folio_test_anon(folio))
> - entry = make_readable_exclusive_migration_entry(
> - swp_offset(entry));
> - else
> - entry = make_readable_migration_entry(swp_offset(entry));
> + if (!is_writable_device_migration_private_entry(entry)) {
> + if (folio_test_anon(folio))
> + entry = make_readable_exclusive_migration_entry(
> + swp_offset(entry));
> + else
> + entry = make_readable_migration_entry(
> + swp_offset(entry));
> + } else {
> + if (folio_test_anon(folio))
> + entry = make_device_migration_readable_exclusive_migration_entry(
> + swp_offset(entry));
> + else
> + entry = make_readable_migration_device_private_entry(
> + swp_offset(entry));
> + }
> +
> newpte = swp_entry_to_pte(entry);
> if (pte_swp_soft_dirty(oldpte))
> newpte = pte_swp_mksoft_dirty(newpte);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 9146bd084435..e9fe747d3df3 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -112,7 +112,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
> return false;
> entry = pte_to_swp_entry(ptent);
>
> - if (!is_migration_entry(entry))
> + if (!(is_migration_entry(entry)))
> return false;
>
> pfn = swp_offset_pfn(entry);
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 9f91cf85a5be..f5c77dda3359 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -1003,7 +1003,8 @@ struct folio *folio_walk_start(struct folio_walk *fw,
> swp_entry_t entry = pte_to_swp_entry(pte);
>
> if ((flags & FW_MIGRATION) &&
> - is_migration_entry(entry)) {
> + (is_migration_entry(entry) ||
> + is_device_private_migration_entry(entry))) {
> page = pfn_swap_entry_to_page(entry);
> expose_page = false;
> goto found;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index e94500318f92..9642a79cbdb4 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2535,15 +2535,29 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> * pte. do_swap_page() will wait until the migration
> * pte is removed and then restart fault handling.
> */
> - if (writable)
> - entry = make_writable_migration_entry(
> - page_to_pfn(subpage));
> - else if (anon_exclusive)
> - entry = make_readable_exclusive_migration_entry(
> - page_to_pfn(subpage));
> - else
> - entry = make_readable_migration_entry(
> - page_to_pfn(subpage));
> + if (writable) {
> + if (is_device_private_page(subpage))
> + entry = make_writable_migration_device_private_entry(
> + page_to_pfn(subpage));
> + else
> + entry = make_writable_migration_entry(
> + page_to_pfn(subpage));
> + } else if (anon_exclusive) {
> + if (is_device_private_page(subpage))
> + entry = make_device_migration_readable_exclusive_migration_entry(
> + page_to_pfn(subpage));
> + else
> + entry = make_readable_exclusive_migration_entry(
> + page_to_pfn(subpage));
> + } else {
> + if (is_device_private_page(subpage))
> + entry = make_readable_migration_device_private_entry(
> + page_to_pfn(subpage));
> + else
> + entry = make_readable_migration_entry(
> + page_to_pfn(subpage));
> + }
> +
> if (likely(pte_present(pteval))) {
> if (pte_young(pteval))
> entry = make_migration_entry_young(entry);
> --
> 2.34.1
>

Thanks,
Chih-En Lin