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Subject: Re: [PATCH 19/19] Documentation/vm: add pin_user_pages.rst
To:     Ira Weiny <ira.weiny@intel.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Thu, 31 Oct 2019 17:48:36 -0700
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On 10/31/19 4:49 PM, Ira Weiny wrote:
> On Wed, Oct 30, 2019 at 03:49:30PM -0700, John Hubbard wrote:
...
>> +TODO: There is also a special case when the pages are DAX pages: in addition to
>> +the above flags, the caller needs something like a layout lease on the
>> +associated file. This is yet to be implemented. When it is implemented, it's
>> +expected that the lease will be a prerequisite to setting FOLL_LONGTERM.
> 
> For now we probably want to leave this note out until we figure out how this is
> going to work.  Best to say something like:
> 
> Some pages, such as DAX pages, can't be pinned with longterm pins and will
> fail.
> 

OK, I have this wording queued up for the v2 patch:

NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's
because DAX pages do not have a separate page cache, and so "pinning" implies
locking down file system blocks, which is not (yet) supported in that way.


thanks,

John Hubbard
NVIDIA