linux-ext4 - Re: [RFC 03/10] pci/p2pdma: Don't initialise page refcount to one

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Date: Thu, 11 Apr 2024 09:29:27 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, david@...morbit.com, dan.j.williams@...el.com,
	jhubbard@...dia.com, rcampbell@...dia.com, willy@...radead.org,
	linux-fsdevel@...r.kernel.org, jack@...e.cz, djwong@...nel.org,
	hch@....de, david@...hat.com, ruansy.fnst@...itsu.com,
	nvdimm@...ts.linux.dev, linux-xfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, jglisse@...hat.com
Subject: Re: [RFC 03/10] pci/p2pdma: Don't initialise page refcount to one

On Thu, Apr 11, 2024 at 10:57:24AM +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@...dia.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 fa7370f..ab7ef18 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;
>  	}
>  
> +	get_page(virt_to_page(kaddr));
> +

Should this be 

 set_page_count(page, 1)

If the refcount is already known to be 0 ?

> @@ -508,15 +508,15 @@ void free_zone_device_page(struct page *page)
>  	page->mapping = NULL;
>  	page->pgmap->ops->page_free(page);
>  
> -	if (page->pgmap->type != MEMORY_DEVICE_PRIVATE &&
> -	    page->pgmap->type != MEMORY_DEVICE_COHERENT)
> +	if (page->pgmap->type == MEMORY_DEVICE_PRIVATE ||
> +	    page->pgmap->type == MEMORY_DEVICE_COHERENT)
> +		put_dev_pagemap(page->pgmap);

Not related, but we should really be getting rid of this devmap
refcount traffic too, IMHO..

If an implementation wants this then it should hook the page
free/alloc callbacks and do this, not put it in the core code.

Jason