Re: [v6 02/15] mm/huge_memory: add device-private THP support to PMD operations
From: David Hildenbrand
Date: Thu Sep 25 2025 - 05:53:33 EST
On 25.09.25 02:25, Alistair Popple wrote:
On 2025-09-16 at 22:21 +1000, Balbir Singh <balbirs@xxxxxxxxxx> 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
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
Cc: Nico Pache <npache@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
---
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)
I can't think of a better name either although I am curious why open-coding it
was so nasty given we don't have the equivalent for pte entries. Will go read
the previous discussion.
I think for PTEs we just handle all cases (markers, hwpoison etc)
properly, manye not being supported yet on the PMD level. See
copy_nonpresent_pte() as an example.
We don't even have helpers like is_pte_migration_entry().
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)))
Why isn't is_pmd_non_present_folio_entry() used here?
I thought I argued that
if (!pmd_present(pmdval)))
Should be sufficient here in my last review?
We want to detect page tables we can map after all.
--
Cheers
David / dhildenb