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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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References: <20191112000700.3455038-1-jhubbard@nvidia.com>
 <20191112000700.3455038-9-jhubbard@nvidia.com>
 <20191112204338.GE5584@ziepe.ca>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Tue, 12 Nov 2019 14:42:57 -0800
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On 11/12/19 12:43 PM, Jason Gunthorpe wrote:
...
>> -		}
>> +	ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
>> +				    page, vmas, NULL);
>> +	/*
>> +	 * The lifetime of a vaddr_get_pfn() page pin is
>> +	 * userspace-controlled. In the fs-dax case this could
>> +	 * lead to indefinite stalls in filesystem operations.
>> +	 * Disallow attempts to pin fs-dax pages via this
>> +	 * interface.
>> +	 */
>> +	if (ret > 0 && vma_is_fsdax(vmas[0])) {
>> +		ret = -EOPNOTSUPP;
>> +		put_page(page[0]);
>>  	}
> 
> AFAIK this chunk is redundant now as it is some hack to emulate
> FOLL_LONGTERM? So vmas can be deleted too.

Let me first make sure I understand what Dan has in mind for the vma
checking, in the other thread...

> 
> Also unclear why this function has this:
> 
>         up_read(&mm->mmap_sem);
> 
>         if (ret == 1) {
>                 *pfn = page_to_pfn(page[0]);
>                 return 0;
>         }
> 
>         down_read(&mm->mmap_sem);
> 

Yes, that's really odd. It's not good to release and retake the lock
anyway in general (without re-checking things), and  certainly it is
not required to release mmap_sem in order to call page_to_pfn().

I've removed that up_read()/down_read() pair, for v4.


thanks,
-- 
John Hubbard
NVIDIA