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

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Message-ID: <87ttdjxzf9.fsf@nvdebian.thelocal>
Date: Fri, 11 Oct 2024 11:20:03 +1100
From: Alistair Popple <apopple@...dia.com>
To: Logan Gunthorpe <logang@...tatee.com>
Cc: dan.j.williams@...el.com, linux-mm@...ck.org, vishal.l.verma@...el.com,
 dave.jiang@...el.com, bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca,
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Subject: Re: [PATCH 02/12] pci/p2pdma: Don't initialise page refcount to one


Logan Gunthorpe <logang@...tatee.com> writes:

> 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.

Good point.

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

Have moved it there for the next version, thanks!

> I think a similar mistake was made by other recent changes.
>
> Thanks,
>
> Logan