linux-kernel - Re: [PATCH] mm, zone_device: replace {get, put}_zone_device

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Date:   Thu, 27 Apr 2017 09:38:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()
 with a single reference

On Thu, Apr 27, 2017 at 9:33 AM, Logan Gunthorpe <logang@...tatee.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.