Re: [PATCH 02/13] pci/p2pdma: Don't initialise page refcount to one

Christoph Hellwig
2024-06-27T05:31:07Z

On Thu, Jun 27, 2024 at 10:54:17AM +1000, 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 | 2 ++
> mm/memremap.c | 8 ++++----
> mm/mm_init.c | 4 +++-
> 3 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4f47a13..1e9ea32 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -128,6 +128,8 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> goto out;
> }
>
> + set_page_count(virt_to_page(kaddr), 1);

Can we have a comment here? Without that it feels a bit too much like
black magic when reading the code.

> + if (folio->page.pgmap->type == MEMORY_DEVICE_PRIVATE ||
> + folio->page.pgmap->type == MEMORY_DEVICE_COHERENT)
> + put_dev_pagemap(folio->page.pgmap);
> + else if (folio->page.pgmap->type != MEMORY_DEVICE_PCI_P2PDMA)
> /*
> * Reset the refcount to 1 to prepare for handing out the page
> * again.
> */
> folio_set_count(folio, 1);

Where the else if evaluates to MEMORY_DEVICE_FS_DAX ||
MEMORY_DEVICE_GENERIC. Maybe make this a switch statement handling
all cases of the enum to make it clear and have the compiler generate
a warning when a new type is added without being handled here?

> @@ -1014,7 +1015,8 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
> * which will set the page count to 1 when allocating the page.
> */
> if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
> + pgmap->type == MEMORY_DEVICE_COHERENT ||
> + pgmap->type == MEMORY_DEVICE_PCI_P2PDMA)
> set_page_count(page, 0);

Similarly here a switch with explanation of what will be handled and
what not would be nice.