Re: [PATCH v3 10/25] pci/p2pdma: Don't initialise page refcount to one

Bjorn Helgaas
2024-11-22T18:20:14Z

On Fri, Nov 22, 2024 at 12:40:31PM +1100, 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>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Previously suggested tweaks to subject line prefix and content:
https://lore.kernel.org/all/20240629212851.GA1484889@bhelgaas/
https://lore.kernel.org/all/20240910134745.GA577955@bhelgaas/

I had the impression that you agreed there was the potential for some
confusion here, but it doesn't look like it was addressed.

So again, a PCI patch labeled "don't init refcount to one" where the
content initializes the refcount to one in p2pdma.c is still confusing
since (IIUC) the subject line refers to the NON-PCI code.

Maybe some sort of "move refcount init from X to p2pdma" or addition
of *who* is no longer initializing refcount to one would clear this
up.

> ---
>
> Changes since v2:
>
> - Initialise the page refcount for all pages covered by the kaddr
> ---
> drivers/pci/p2pdma.c | 13 +++++++++++--
> mm/memremap.c | 17 +++++++++++++----
> mm/mm_init.c | 22 ++++++++++++++++++----
> 3 files changed, 42 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4f47a13..2c5ac4a 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -140,13 +140,22 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> rcu_read_unlock();
>
> for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) {
> - ret = vm_insert_page(vma, vaddr, virt_to_page(kaddr));
> + struct page *page = virt_to_page(kaddr);
> +
> + /*
> + * Initialise the refcount for the freshly allocated page. As
> + * we have just allocated the page no one else should be
> + * using it.
> + */
> + VM_WARN_ON_ONCE_PAGE(!page_ref_count(page), page);
> + set_page_count(page, 1);
> + ret = vm_insert_page(vma, vaddr, page);
> if (ret) {
> gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> return ret;
> }
> percpu_ref_get(ref);
> - put_page(virt_to_page(kaddr));
> + put_page(page);
> kaddr += PAGE_SIZE;
> len -= PAGE_SIZE;
> }
> 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;
> + }
> }
>
> 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).
> */
> - if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
> - pgmap->type == MEMORY_DEVICE_COHERENT)
> + switch (pgmap->type) {
> + case MEMORY_DEVICE_PRIVATE:
> + case MEMORY_DEVICE_COHERENT:
> + case MEMORY_DEVICE_PCI_P2PDMA:
> set_page_count(page, 0);
> + break;
> +
> + case MEMORY_DEVICE_FS_DAX:
> + case MEMORY_DEVICE_GENERIC:
> + break;
> + }
> }
>
> /*
> --
> git-series 0.9.1