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Date: Sat, 8 Dec 2018 13:45:26 -0500
From: Jerome Glisse <jglisse@redhat.com>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Message-ID: <20181208184525.GC2952@redhat.com>
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To: Christoph Hellwig <hch@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>, John Hubbard <jhubbard@nvidia.com>, Matthew Wilcox <willy@infradead.org>, John Hubbard <john.hubbard@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Linux MM <linux-mm@kvack.org>, Jan Kara <jack@suse.cz>, tom@talpey.com, Al Viro <viro@zeniv.linux.org.uk>, benve@cisco.com, Christopher Lameter <cl@linux.com>, "Dalessandro, Dennis" <dennis.dalessandro@intel.com>, Doug Ledford <dledford@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>, Mike Marciniszyn <mike.marciniszyn@intel.com>, rcampbell@nvidia.com, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>

On Sat, Dec 08, 2018 at 10:26:04AM -0800, Christoph Hellwig wrote:
> On Sat, Dec 08, 2018 at 12:47:30PM -0500, Jerome Glisse wrote:
> > Most of the user of GUP are well behave (everything under driver/gpu and
> > so is mellanox driver and many other) ie they abide by mmu notifier
> > invalidation call backs. They are a handfull of device driver that thought
> > they could just do GUP and ignore the mmu notifier part and those are the
> > one being problematic. So to me it feels like bystander are be shot for no
> > good reasons.
> 
> get_user_pages is used by every single direct I/O, and while the race
> windows in that case are small they very much exists.

Yes and my proposal allow to fix that in even a better way than
the pin count would ie allowing to provide a callback for write
back to wait on direct I/O as you said for direct I/O it is a
small window so it would be fine to have write back wait on it.

Cheers,
J�r�me