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

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Date:   Thu, 27 Apr 2017 09:14:09 -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 9:11 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 26/04/17 06:55 PM, Dan Williams wrote:
>> @@ -277,7 +269,10 @@ struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
>>   *
>>   * Notes:
>>   * 1/ @ref must be 'live' on entry and 'dead' before devm_memunmap_pages() time
>> - *    (or devm release event).
>> + *    (or devm release event). The expected order of events is that @ref has
>> + *    been through percpu_ref_kill() before devm_memremap_pages_release(). The
>> + *    wait for the completion of kill and percpu_ref_exit() must occur after
>> + *    devm_memremap_pages_release().
>>   *
>>   * 2/ @res is expected to be a host memory range that could feasibly be
>>   *    treated as a "System RAM" range, i.e. not a device mmio range, but
>> @@ -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);
>>       }
>>       devres_add(dev, page_map);
>>       return __va(res->start);
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 5dabf444d724..01267dda6668 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -97,6 +97,16 @@ static void __put_compound_page(struct page *page)
>>
>>  void __put_page(struct page *page)
>>  {
>> +     if (is_zone_device_page(page)) {
>> +             put_dev_pagemap(page->pgmap);
>> +
>> +             /*
>> +              * The page belong to device, do not return it to
>> +              * page allocator.
>> +              */
>> +             return;
>> +     }
>> +
>>       if (unlikely(PageCompound(page)))
>>               __put_compound_page(page);
>>       else
>>
>
> Forgive me if I'm missing something but this doesn't make sense to me.
> We are taking a reference once when the region is initialized and
> releasing it every time a page within the region's reference count drops
> to zero. That does not seem to be symmetric and I don't see how it
> tracks that pages are in use. Shouldn't get_dev_pagemap be called when
> any page is allocated or something like that (ie. the inverse of
> __put_page)?

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.