netdev - Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL

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Message-ID: <0db36e86-b779-01af-77e7-469af2a2e19c@nvidia.com>
Date:   Tue, 12 Nov 2019 14:42:57 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM

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