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Date: Tue, 4 Dec 2018 17:15:19 -0800
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Message-ID: <20181205011519.GV10377@bombadil.infradead.org>
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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, Christoph Hellwig <hch@infradead.org>, Christopher Lameter <cl@linux.com>, "Dalessandro, Dennis" <dennis.dalessandro@intel.com>, Doug Ledford <dledford@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>, =?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>

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?