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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 27 Apr 2017 09:38:29 -0700
Message-ID: <CAPcyv4iFKb9VbwdjRwF4KLQ=2R6-=vYb6BbHQG6Kk-8QemC6WA@mail.gmail.com>
Subject: Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()
 with a single reference
To: Logan Gunthorpe <logang@deltatee.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>
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On Thu, Apr 27, 2017 at 9:33 AM, Logan Gunthorpe <logang@deltatee.com> wrote:
>
>
> On 27/04/17 10:14 AM, Dan Williams wrote:
>> You're overlooking that the page reference count 1 after
>> arch_add_memory(). So at the end of time we're just dropping the
>> arch_add_memory() reference to release the page and related
>> dev_pagemap.
>
> Thanks, that does actually make a lot more sense to me now. However,
> there still appears to be an asymmetry in that the pgmap->ref is
> incremented once and decremented once per page...
>

No, this hunk...

@@ -379,6 +374,7 @@ void *devm_memremap_pages(struct device *dev,
struct resource *res,
                 */
                list_del(&page->lru);
                page->pgmap = pgmap;
+               percpu_ref_get(ref);
        }


...is inside a for_each_device_pfn() loop.