Re: [PATCH 02/12] pci/p2pdma: Don't initialise page refcount to one

Dan Williams
2024-09-22T01:00:58Z

Alistair Popple wrote:
> The reference counts for ZONE_DEVICE private pages should be
> initialised by the driver when the page is actually allocated by the
> driver allocator, not when they are first created. This is currently
> the case for MEMORY_DEVICE_PRIVATE and MEMORY_DEVICE_COHERENT pages
> but not MEMORY_DEVICE_PCI_P2PDMA pages so fix that up.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> drivers/pci/p2pdma.c | 6 ++++++
> mm/memremap.c | 17 +++++++++++++----
> mm/mm_init.c | 22 ++++++++++++++++++----
> 3 files changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4f47a13..210b9f4 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -129,6 +129,12 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> }
>
> /*
> + * Initialise the refcount for the freshly allocated page. As we have
> + * just allocated the page no one else should be using it.
> + */
> + set_page_count(virt_to_page(kaddr), 1);

Perhaps VM_WARN_ONCE to back up that assumption?

I also notice that there are multiple virt_to_page() lookups in this
routine, so maybe time for a local @page variable.

> +
> + /*
> * vm_insert_page() can sleep, so a reference is taken to mapping
> * such that rcu_read_unlock() can be done before inserting the
> * pages
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 40d4547..07bbe0e 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -488,15 +488,24 @@ void free_zone_device_folio(struct folio *folio)
> folio->mapping = NULL;
> folio->page.pgmap->ops->page_free(folio_page(folio, 0));
>
> - if (folio->page.pgmap->type != MEMORY_DEVICE_PRIVATE &&
> - folio->page.pgmap->type != MEMORY_DEVICE_COHERENT)
> + switch (folio->page.pgmap->type) {
> + case MEMORY_DEVICE_PRIVATE:
> + case MEMORY_DEVICE_COHERENT:
> + put_dev_pagemap(folio->page.pgmap);
> + break;
> +
> + case MEMORY_DEVICE_FS_DAX:
> + case MEMORY_DEVICE_GENERIC:
> /*
> * Reset the refcount to 1 to prepare for handing out the page
> * again.
> */
> folio_set_count(folio, 1);
> - else
> - put_dev_pagemap(folio->page.pgmap);
> + break;
> +
> + case MEMORY_DEVICE_PCI_P2PDMA:
> + break;

A follow on cleanup is that either all implementations should be
put_dev_pagemap(), or none of them. Put the onus on the implementation
to track how many pages it has handed out in the implementation
allocator.

> + }
> }
>
> void zone_device_page_init(struct page *page)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 4ba5607..0489820 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1015,12 +1015,26 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
> }
>
> /*
> - * ZONE_DEVICE pages are released directly to the driver page allocator
> - * which will set the page count to 1 when allocating the page.
> + * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC and
> + * MEMORY_TYPE_FS_DAX pages are released directly to the driver page
> + * allocator which will set the page count to 1 when allocating the
> + * page.
> + *
> + * MEMORY_TYPE_GENERIC and MEMORY_TYPE_FS_DAX pages automatically have
> + * their refcount reset to one whenever they are freed (ie. after
> + * their refcount drops to 0).

I'll send some follow on patches to clean up device-dax.

For this one:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>