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

From: Logan Gunthorpe <logang@deltatee.com>
To: Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org
Cc: david@fromorbit.com, dan.j.williams@intel.com,
	jhubbard@nvidia.com, rcampbell@nvidia.com, willy@infradead.org,
	jgg@nvidia.com, linux-fsdevel@vger.kernel.org, jack@suse.cz,
	djwong@kernel.org, hch@lst.de, david@redhat.com,
	ruansy.fnst@fujitsu.com, nvdimm@lists.linux.dev,
	linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
	jglisse@redhat.com
Subject: Re: [RFC 03/10] pci/p2pdma: Don't initialise page refcount to one
Date: Thu, 9 May 2024 15:59:35 -0600	[thread overview]
Message-ID: <4393e0d9-28c5-4b85-b603-40e457c9beba@deltatee.com> (raw)
In-Reply-To: <a443974e64917824e078485d4e755ef04c89d73f.1712796818.git-series.apopple@nvidia.com>

Hi Alistair,

I was working on testing your patch set, however I'm dealing with some
hardware issues at the moment so I haven't fully tested everything yet.

I managed to find one issue though:

On 2024-04-10 18:57, Alistair Popple wrote:
> 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));
> +

kaddr may represent more than one page, so this will fail to map
anything if the mapping size is greater than 4KB. There is a loop just
below this that calls vm_insert_page(). Moving a set_page_count() call
just before vm_insert_page() fixes the issue.

Thanks!

Logan