linux-kernel - [v1 05/12] mm/memory/fault: add support for zone device THP fault handling

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Message-ID: <20250703222759.1943776-6-balbirs@nvidia.com>
Date: Fri,  4 Jul 2025 08:27:52 +1000
From: Balbir Singh <balbirs@...dia.com>
To: linux-mm@...ck.org
Cc: akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Balbir Singh <balbirs@...dia.com>
Subject: [v1 05/12] mm/memory/fault: add support for zone device THP fault handling

When the CPU touches a zone device THP entry, the data needs to
be migrated back to the CPU, call migrate_to_ram() on these pages
via do_huge_pmd_device_private() fault handling helper.

Signed-off-by: Balbir Singh <balbirs@...dia.com>
---
 include/linux/huge_mm.h |  7 +++++++
 mm/huge_memory.c        | 40 ++++++++++++++++++++++++++++++++++++++++
 mm/memory.c             |  6 ++++--
 3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 4d5bb67dc4ec..65a1bdf29bb9 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -474,6 +474,8 @@ static inline bool folio_test_pmd_mappable(struct folio *folio)
 
 vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
 
+vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf);
+
 extern struct folio *huge_zero_folio;
 extern unsigned long huge_zero_pfn;
 
@@ -627,6 +629,11 @@ static inline vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 	return 0;
 }
 
+static inline vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
+{
+	return 0;
+}
+
 static inline bool is_huge_zero_folio(const struct folio *folio)
 {
 	return false;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e6e390d0308f..f29add796931 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1267,6 +1267,46 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 
 }
 
+vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	vm_fault_t ret = 0;
+	spinlock_t *ptl;
+	swp_entry_t swp_entry;
+	struct page *page;
+
+	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
+		return VM_FAULT_FALLBACK;
+
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+		vma_end_read(vma);
+		return VM_FAULT_RETRY;
+	}
+
+	ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+	if (unlikely(!pmd_same(*vmf->pmd, vmf->orig_pmd))) {
+		spin_unlock(ptl);
+		return 0;
+	}
+
+	swp_entry = pmd_to_swp_entry(vmf->orig_pmd);
+	page = pfn_swap_entry_to_page(swp_entry);
+	vmf->page = page;
+	vmf->pte = NULL;
+	if (trylock_page(vmf->page)) {
+		get_page(page);
+		spin_unlock(ptl);
+		ret = page_pgmap(page)->ops->migrate_to_ram(vmf);
+		unlock_page(vmf->page);
+		put_page(page);
+	} else {
+		spin_unlock(ptl);
+	}
+
+	return ret;
+}
+
 /*
  * always: directly stall for all thp allocations
  * defer: wake kswapd and fail if not immediately available
diff --git a/mm/memory.c b/mm/memory.c
index 0f9b32a20e5b..c26c421b8325 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6165,8 +6165,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
 		vmf.orig_pmd = pmdp_get_lockless(vmf.pmd);
 
 		if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
-			VM_BUG_ON(thp_migration_supported() &&
-					  !is_pmd_migration_entry(vmf.orig_pmd));
+			if (is_device_private_entry(
+					pmd_to_swp_entry(vmf.orig_pmd)))
+				return do_huge_pmd_device_private(&vmf);
+
 			if (is_pmd_migration_entry(vmf.orig_pmd))
 				pmd_migration_entry_wait(mm, vmf.pmd);
 			return 0;
-- 
2.49.0