linux-kernel - [v2 04/11] mm/memory/fault: add support for zone device THP fault handling

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Message-ID: <20250730092139.3890844-5-balbirs@nvidia.com>
Date: Wed, 30 Jul 2025 19:21:32 +1000
From: Balbir Singh <balbirs@...dia.com>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	Balbir Singh <balbirs@...dia.com>,
	Karol Herbst <kherbst@...hat.com>,
	Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Jérôme Glisse <jglisse@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	David Hildenbrand <david@...hat.com>,
	Barry Song <baohua@...nel.org>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Xu <peterx@...hat.com>,
	Zi Yan <ziy@...dia.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Jane Chu <jane.chu@...cle.com>,
	Alistair Popple <apopple@...dia.com>,
	Donet Tom <donettom@...ux.ibm.com>,
	Ralph Campbell <rcampbell@...dia.com>,
	Mika Penttilä <mpenttil@...hat.com>,
	Matthew Brost <matthew.brost@...el.com>,
	Francois Dugast <francois.dugast@...el.com>
Subject: [v2 04/11] 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.

Cc: Karol Herbst <kherbst@...hat.com>
Cc: Lyude Paul <lyude@...hat.com>
Cc: Danilo Krummrich <dakr@...nel.org>
Cc: David Airlie <airlied@...il.com>
Cc: Simona Vetter <simona@...ll.ch>
Cc: "Jérôme Glisse" <jglisse@...hat.com>
Cc: Shuah Khan <shuah@...nel.org>
Cc: David Hildenbrand <david@...hat.com>
Cc: Barry Song <baohua@...nel.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Ryan Roberts <ryan.roberts@....com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Peter Xu <peterx@...hat.com>
Cc: Zi Yan <ziy@...dia.com>
Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
Cc: Jane Chu <jane.chu@...cle.com>
Cc: Alistair Popple <apopple@...dia.com>
Cc: Donet Tom <donettom@...ux.ibm.com>
Cc: Ralph Campbell <rcampbell@...dia.com>
Cc: Mika Penttilä <mpenttil@...hat.com>
Cc: Matthew Brost <matthew.brost@...el.com>
Cc: Francois Dugast <francois.dugast@...el.com>

Signed-off-by: Balbir Singh <balbirs@...dia.com>
---
 include/linux/huge_mm.h |  7 +++++++
 mm/huge_memory.c        | 36 ++++++++++++++++++++++++++++++++++++
 mm/memory.c             |  6 ++++--
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2a6f5ff7bca3..56fdcaf7604b 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -475,6 +475,8 @@ 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;
 
@@ -633,6 +635,11 @@ 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 e373c6578894..713dd433d352 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1271,6 +1271,42 @@ 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;
+	vm_fault_t ret = 0;
+	spinlock_t *ptl;
+	swp_entry_t swp_entry;
+	struct page *page;
+
+	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 92fd18a5d8d1..6c87f043eea1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6152,8 +6152,10 @@ 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;
-- 
2.50.1