Re: [PATCH v4 10/25] mm/mm_init: Move p2pdma page refcount initialisation to p2pdma

[David Hildenbrand <david@redhat.com>]

On 17.12.24 06:12, Alistair Popple wrote:
> Currently ZONE_DEVICE page reference counts are initialised by core
> memory management code in __init_zone_device_page() as part of the
> memremap() call which driver modules make to obtain ZONE_DEVICE
> pages. This initialises page refcounts to 1 before returning them to
> the driver.
> 
> This was presumably done because it drivers had a reference of sorts
> on the page. It also ensured the page could always be mapped with
> vm_insert_page() for example and would never get freed (ie. have a
> zero refcount), freeing drivers of manipulating page reference counts.

It probably dates back to copying that code from other zone-init code 
where we
(a) Treat all available-at-boot memory as allocated before we release it 
to the buddy
(b) Treat all hotplugged memory as allocated until we release it to the 
buddy

As a side note, I'm working on converting (b) -- PageOffline pages -- to 
have a refcount of 0 ("frozen").

> 
> However it complicates figuring out whether or not a page is free from
> the mm perspective because it is no longer possible to just look at
> the refcount. Instead the page type must be known and if GUP is used a
> secondary pgmap reference is also sometimes needed.
> 
> To simplify this it is desirable to remove the page reference count
> for the driver, so core mm can just use the refcount without having to
> account for page type or do other types of tracking. This is possible
> because drivers can always assume the page is valid as core kernel
> will never offline or remove the struct page.
> 
> This means it is now up to drivers to initialise the page refcount as
> required. P2PDMA uses vm_insert_page() to map the page, and that
> requires a non-zero reference count when initialising the page so set
> that when the page is first mapped.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> 
> ---
> 
> 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 0cb7e0a..04773a8 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 24b68b4..f021e63 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1017,12 +1017,26 @@ 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;
> +	}
>   }
>   
>   /*


But that's a bit weird: we call __init_single_page()->init_page_count() 
to initialize it to 1, to then set it back to 0.


Maybe we can just pass to __init_single_page() the refcount we want to 
have directly? Can be a patch on top of course.

Apart from that

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb