[PATCH v2 2/8] mm: Free device private pages have zero refcount

[Alistair Popple]

Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page
refcount") device private pages have no longer had an extra reference
count when the page is in use. However before handing them back to the
owning device driver we add an extra reference count such that free
pages have a reference count of one.

This makes it difficult to tell if a page is free or not because both
free and in use pages will have a non-zero refcount. Instead we should
return pages to the drivers page allocator with a zero reference count.
Kernel code can then safely use kernel functions such as
get_page_unless_zero().

Signed-off-by: Alistair Popple <apopple at nvidia.com>
Cc: Jason Gunthorpe <jgg at nvidia.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Felix Kuehling <Felix.Kuehling at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: Christian König <christian.koenig at amd.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: Lyude Paul <lyude at redhat.com>
Cc: Ralph Campbell <rcampbell at nvidia.com>
Cc: Alex Sierra <alex.sierra at amd.com>
Cc: John Hubbard <jhubbard at nvidia.com>
Cc: Dan Williams <dan.j.williams at intel.com>

---

This will conflict with Dan's series to fix reference counts for DAX[1].
At the moment this only makes changes for device private and coherent
pages, however if DAX is fixed to remove the extra refcount then we
should just be able to drop the checks for private/coherent pages and
treat them the same.

[1] - https://lore.kernel.org/linux-mm/166329930818.2786261.6086109734008025807.stgit@dwillia2-xfh.jf.intel.com/
---
 arch/powerpc/kvm/book3s_hv_uvmem.c       |  2 +-
 drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  2 +-
 drivers/gpu/drm/nouveau/nouveau_dmem.c   |  2 +-
 include/linux/memremap.h                 |  1 +
 lib/test_hmm.c                           |  2 +-
 mm/memremap.c                            |  9 +++++++++
 mm/page_alloc.c                          |  8 ++++++++
 7 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index d4eacf4..9d8de68 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -718,7 +718,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
 
 	dpage = pfn_to_page(uvmem_pfn);
 	dpage->zone_device_data = pvt;
-	lock_page(dpage);
+	zone_device_page_init(dpage);
 	return dpage;
 out_clear:
 	spin_lock(&kvmppc_uvmem_bitmap_lock);
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
index 776448b..97a6845 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
@@ -223,7 +223,7 @@ svm_migrate_get_vram_page(struct svm_range *prange, unsigned long pfn)
 	page = pfn_to_page(pfn);
 	svm_range_bo_ref(prange->svm_bo);
 	page->zone_device_data = prange->svm_bo;
-	lock_page(page);
+	zone_device_page_init(page);
 }
 
 static void
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index 1635661..b092988 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -326,7 +326,7 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm)
 			return NULL;
 	}
 
-	lock_page(page);
+	zone_device_page_init(page);
 	return page;
 }
 
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 1901049..f68bf6d 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -182,6 +182,7 @@ static inline bool folio_is_device_coherent(const struct folio *folio)
 }
 
 #ifdef CONFIG_ZONE_DEVICE
+void zone_device_page_init(struct page *page);
 void *memremap_pages(struct dev_pagemap *pgmap, int nid);
 void memunmap_pages(struct dev_pagemap *pgmap);
 void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 89463ff..688c15d 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -627,8 +627,8 @@ static struct page *dmirror_devmem_alloc_page(struct dmirror_device *mdevice)
 			goto error;
 	}
 
+	zone_device_page_init(dpage);
 	dpage->zone_device_data = rpage;
-	lock_page(dpage);
 	return dpage;
 
 error:
diff --git a/mm/memremap.c b/mm/memremap.c
index 25029a4..1c2c038 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -505,8 +505,17 @@ void free_zone_device_page(struct page *page)
 	/*
 	 * Reset the page count to 1 to prepare for handing out the page again.
 	 */
+	if (page->pgmap->type != MEMORY_DEVICE_PRIVATE &&
+	    page->pgmap->type != MEMORY_DEVICE_COHERENT)
+		set_page_count(page, 1);
+}
+
+void zone_device_page_init(struct page *page)
+{
 	set_page_count(page, 1);
+	lock_page(page);
 }
+EXPORT_SYMBOL_GPL(zone_device_page_init);
 
 #ifdef CONFIG_FS_DAX
 bool __put_devmap_managed_page_refs(struct page *page, int refs)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9d49803..4df1e43 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6744,6 +6744,14 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
 		set_pageblock_migratetype(page, MIGRATE_MOVABLE);
 		cond_resched();
 	}
+
+	/*
+	 * ZONE_DEVICE pages are released directly to the driver page allocator
+	 * which will set the page count to 1 when allocating the page.
+	 */
+	if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
+	    pgmap->type == MEMORY_DEVICE_COHERENT)
+		set_page_count(page, 0);
 }
 
 /*
-- 
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