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

David Hildenbrand (Arm)
2026-03-06T16:04:12Z

On 3/6/26 17:02, David Hildenbrand (Arm) wrote:
> 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?
>

Or maybe even better, if the pfn has no meaning, set it to 0 or -1, or
anything that will make the other code just ignore it.

--
Cheers,

David