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

From: Bjorn Helgaas <[email-masked]>
Date: Fri, 22 Nov 2024 12:20:13 -0600

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 <[email-masked]>
> Reviewed-by: Dan Williams <[email-masked]>

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