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

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Message-ID: <28355eb0-4ee5-3418-b430-59302d15b478@nvidia.com>
Date:   Tue, 12 Nov 2019 17:08:07 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        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>,
        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>,
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        Michal Hocko <mhocko@...e.com>,
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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM

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).

thanks,
-- 
John Hubbard
NVIDIA