linux-kernel - Re: [PATCH 02/12] pci/p2pdma: Don't initialise page refcount to one

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Message-ID: <6f3402ae-01ad-4764-8941-f88bc77f5227@deltatee.com>
Date: Tue, 10 Sep 2024 18:48:48 -0600
From: Logan Gunthorpe <logang@...tatee.com>
To: Alistair Popple <apopple@...dia.com>, dan.j.williams@...el.com,
 linux-mm@...ck.org
Cc: vishal.l.verma@...el.com, dave.jiang@...el.com, bhelgaas@...gle.com,
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 mpe@...erman.id.au, npiggin@...il.com, dave.hansen@...ux.intel.com,
 ira.weiny@...el.com, willy@...radead.org, djwong@...nel.org, tytso@....edu,
 linmiaohe@...wei.com, david@...hat.com, peterx@...hat.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
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 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 david@...morbit.com
Subject: Re: [PATCH 02/12] pci/p2pdma: Don't initialise page refcount to one



On 2024-09-09 22:14, 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 |  6 ++++++
>  mm/memremap.c        | 17 +++++++++++++----
>  mm/mm_init.c         | 22 ++++++++++++++++++----
>  3 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4f47a13..210b9f4 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -129,6 +129,12 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
>  	}
>  
>  	/*
> +	 * Initialise the refcount for the freshly allocated page. As we have
> +	 * just allocated the page no one else should be using it.
> +	 */
> +	set_page_count(virt_to_page(kaddr), 1);
> +
> +	/*
>  	 * vm_insert_page() can sleep, so a reference is taken to mapping
>  	 * such that rcu_read_unlock() can be done before inserting the
>  	 * pages
This seems to only set reference count to the first page, when there can
be more than one page referenced by kaddr.

I suspect the page count adjustment should be done in the for loop
that's a few lines lower than this.

I think a similar mistake was made by other recent changes.

Thanks,

Logan