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

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Date: Thu, 27 Jun 2024 07:30:59 +0200
From: Christoph Hellwig <hch@....de>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
	dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
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	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
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	jhubbard@...dia.com, hch@....de, david@...morbit.com
Subject: Re: [PATCH 02/13] pci/p2pdma: Don't initialise page refcount to one

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@...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 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.