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

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Message-ID: <85987617-9f6b-6bd3-fea2-9f2910d942bd@nvidia.com>
Date:   Tue, 12 Nov 2019 15:17:22 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
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        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM

On 11/12/19 2:45 PM, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> 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...
> 
> It's not redundant relative to upstream which does not do anything the
> FOLL_LONGTERM in the gup-slow path... but I have not looked at patches
> 1-7 to see if something there made it redundant.
> 

There is nothing in patches 1-7 that would make it redundant. 

About the only thing that you might find interesting in that subset is
patch 4 ("mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages"),
for devmap and ZONE_DEVICE interest. But it doesn't affect this
discussion directly.


thanks,
-- 
John Hubbard
NVIDIA