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Date:   Fri, 15 Nov 2019 10:08:42 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alex Williamson <alex.williamson@redhat.com>,
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        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Dave Chinner <david@fromorbit.com>,
        David Airlie <airlied@linux.ie>,
        "David S . Miller" <davem@davemloft.net>,
        Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>,
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        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
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Subject: Re: [PATCH v5 09/24] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM
Message-ID: <20191115140842.GA4055@ziepe.ca>
References: <20191115055340.1825745-1-jhubbard@nvidia.com>
 <20191115055340.1825745-10-jhubbard@nvidia.com>
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On Thu, Nov 14, 2019 at 09:53:25PM -0800, John Hubbard wrote:
> As it says in the updated comment in gup.c: current FOLL_LONGTERM
> behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
> FS DAX check requirement on vmas.
> 
> However, the corresponding restriction in get_user_pages_remote() was
> slightly stricter than is actually required: it forbade all
> FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
> that do not set the "locked" arg.
> 
> Update the code and comments accordingly, and update the VFIO caller
> to take advantage of this, fixing a bug as a result: the VFIO caller
> is logically a FOLL_LONGTERM user.
> 
> Also, remove an unnessary pair of calls that were releasing and
> reacquiring the mmap_sem. There is no need to avoid holding mmap_sem
> just in order to call page_to_pfn().
> 
> Also, move the DAX check ("if a VMA is DAX, don't allow long term
> pinning") from the VFIO call site, all the way into the internals
> of get_user_pages_remote() and __gup_longterm_locked(). That is:
> get_user_pages_remote() calls __gup_longterm_locked(), which in turn
> calls check_dax_vmas(). It's lightly explained in the comments as well.
> 
> Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
> and to Dan Williams for helping clarify the DAX refactoring.
> 
> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Jerome Glisse <jglisse@redhat.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 30 +++++-------------------------
>  mm/gup.c                        | 27 ++++++++++++++++++++++-----
>  2 files changed, 27 insertions(+), 30 deletions(-)

Looks OK now  

Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>