Re: [PATCH v6 11/13] mm/util: Add flag to track device private pages in page snapshots

David Hildenbrand (Arm)
2026-03-06T16:03:05Z

On 2/2/26 12:36, 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 pfns and must be handled separately.
>
> Add a new flag PAGE_SNAPSHOT_DEVICE_PRIVATE to track when the pfn of a
> page snapshot is a device private page.
>
> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> v1:
> - No change
> v2:
> - No change
> v3:
> - No change
> v4:
> - Move logical continuation to previous line
> ---
> fs/proc/page.c | 6 ++++--
> include/linux/mm.h | 7 ++++---
> mm/util.c | 3 +++
> 3 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index f9b2c2c906cd..bc14f7ebc369 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -191,10 +191,12 @@ u64 stable_page_flags(const struct page *page)
> folio_test_large_rmappable(folio)) {
> /* Note: we indicate any THPs here, not just PMD-sized ones */
> u |= 1 << KPF_THP;
> - } else if (is_huge_zero_pfn(ps.pfn)) {
> + } else if (!(ps.flags & PAGE_SNAPSHOT_DEVICE_PRIVATE) &&
> + is_huge_zero_pfn(ps.pfn)) {
> u |= 1 << KPF_ZERO_PAGE;
> u |= 1 << KPF_THP;
> - } else if (is_zero_pfn(ps.pfn)) {
> + } else if (!(ps.flags & PAGE_SNAPSHOT_DEVICE_PRIVATE) &&
> + is_zero_pfn(ps.pfn)) {
> u |= 1 << KPF_ZERO_PAGE;
> }
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f0d5be9dc736..a52979536a5e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4627,9 +4627,10 @@ static inline bool page_pool_page_is_pp(const struct page *page)
> }
> #endif
>
> -#define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
> -#define PAGE_SNAPSHOT_PG_BUDDY (1 << 1)
> -#define PAGE_SNAPSHOT_PG_IDLE (1 << 2)
> +#define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
> +#define PAGE_SNAPSHOT_PG_BUDDY (1 << 1)
> +#define PAGE_SNAPSHOT_PG_IDLE (1 << 2)
> +#define PAGE_SNAPSHOT_DEVICE_PRIVATE (1 << 3)
>
> struct page_snapshot {
> struct folio folio_snapshot;
> diff --git a/mm/util.c b/mm/util.c
> index 97cae40c0209..65e3f1a97d76 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1218,6 +1218,9 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
>
> if (folio_test_idle(folio))
> ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
> +
> + if (is_device_private_page(page))
> + ps->flags |= PAGE_SNAPSHOT_DEVICE_PRIVATE;

stable_page_flags() has access to the page and can simply test that instead?

--
Cheers,

David