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Date: Tue, 4 Dec 2018 17:15:19 -0800
From: Matthew Wilcox <willy@infradead.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	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,
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	Christopher Lameter <cl@linux.com>,
	"Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
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	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Michal Hocko <mhocko@kernel.org>, mike.marciniszyn@intel.com,
	rcampbell@nvidia.com,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
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On Tue, Dec 04, 2018 at 04:58:01PM -0800, John Hubbard wrote:
> On 12/4/18 3:03 PM, Dan Williams wrote:
> > Except the LRU fields are already in use for ZONE_DEVICE pages... how
> > does this proposal interact with those?
> 
> Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
> use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
> way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
> 
> If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole 
> LRU field approach is unusable.

We just need to rearrange ZONE_DEVICE pages.  Please excuse the whitespace
damage:

+++ b/include/linux/mm_types.h
@@ -151,10 +151,12 @@ struct page {
 #endif
                };
                struct {        /* ZONE_DEVICE pages */
+                       unsigned long _zd_pad_2;        /* LRU */
+                       unsigned long _zd_pad_3;        /* LRU */
+                       unsigned long _zd_pad_1;        /* uses mapping */
                        /** @pgmap: Points to the hosting device page map. */
                        struct dev_pagemap *pgmap;
                        unsigned long hmm_data;
-                       unsigned long _zd_pad_1;        /* uses mapping */
                };
 
                /** @rcu_head: You can use this to free a page by RCU. */

You don't use page->private or page->index, do you Dan?