Nouveau - Sep 2022 - [PATCH 2/7] mm: Free device private pages have zero refcount

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>
---
 arch/powerpc/kvm/book3s_hv_uvmem.c       | 1 +
 drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 +
 drivers/gpu/drm/nouveau/nouveau_dmem.c   | 1 +
 lib/test_hmm.c                           | 1 +
 mm/memremap.c                            | 5 -----
 mm/page_alloc.c                          | 6 ++++++
 6 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c
b/arch/powerpc/kvm/book3s_hv_uvmem.c
index d4eacf4..08d2f7d 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -718,6 +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;
+	set_page_count(dpage, 1);
 	lock_page(dpage);
 	return dpage;
 out_clear:
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
index 776448b..05c2f4d 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
@@ -223,6 +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;
+	set_page_count(page, 1);
 	lock_page(page);
 }
 
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c
b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index 1635661..f9234ed 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -326,6 +326,7 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm)
 			return NULL;
 	}
 
+	set_page_count(page, 1);
 	lock_page(page);
 	return page;
 }
diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 89463ff..2bd3a67 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -627,6 +627,7 @@ static struct page *dmirror_devmem_alloc_page(struct
dmirror_device *mdevice)
 			goto error;
 	}
 
+	set_page_count(dpage, 1);
 	dpage->zone_device_data = rpage;
 	lock_page(dpage);
 	return dpage;
diff --git a/mm/memremap.c b/mm/memremap.c
index 25029a4..e065171 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -501,11 +501,6 @@ void free_zone_device_page(struct page *page)
 	 */
 	page->mapping = NULL;
 	page->pgmap->ops->page_free(page);
-
-	/*
-	 * Reset the page count to 1 to prepare for handing out the page again.
-	 */
-	set_page_count(page, 1);
 }
 
 #ifdef CONFIG_FS_DAX
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9d49803..67eaab5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6744,6 +6744,12 @@ 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.
+	 */
+	set_page_count(page, 0);
 }
 
 /*
-- 
git-series 0.9.1

On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple
wrote:
> 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>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c       | 1 +
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 +
>  drivers/gpu/drm/nouveau/nouveau_dmem.c   | 1 +
>  lib/test_hmm.c                           | 1 +
>  mm/memremap.c                            | 5 -----
>  mm/page_alloc.c                          | 6 ++++++
>  6 files changed, 10 insertions(+), 5 deletions(-)
I think this is a great idea, but I'm surprised no dax stuff is
touched here?

Jason