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

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Message-ID: <c6f035f5-8290-2bc0-a645-d63e3a47f588@nvidia.com>
Date:   Tue, 12 Nov 2019 18:09:08 -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>,
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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM

On 11/12/19 5:35 PM, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 5:08 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> On 11/12/19 4:58 PM, Dan Williams wrote:
>> ...
>>>>> 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.
>>>>
>>>> Oh, the hunk John had below for get_user_pages_remote() also needs to
>>>> call __gup_longterm_locked() when FOLL_LONGTERM is specified, then
>>>> that calls check_dax_vmas() which duplicates the vma_is_fsdax() check
>>>> above.
>>>
>>> Oh true, good eye. It is redundant if it does additionally call
>>> __gup_longterm_locked(), and it needs to do that otherwises it undoes
>>> the CMA migration magic that Aneesh added.
>>>
>>
>> OK. So just to be clear, I'll be removing this from the patch:
>>
>>         /*
>>          * 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]);
>>         }
>>
>> (and the declaration of "vmas", as well).
> 
> ...and add a call to __gup_longterm_locked internal to
> get_user_pages_remote(), right?
> 

Yes, and thanks for double-checking. I think I got a little dizzy following
the call stack there. :)  And now I see that this also affects the
implementation of pin_longterm_pages_remote(), because that will need the
same logic that get_user_pages_remote() has. 



thanks,
-- 
John Hubbard
NVIDIA