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From: Dan Williams <dan.j.williams@intel.com>
Date: Sun, 30 Apr 2017 19:45:08 -0700
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Subject: Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()
 with a single reference
To: Ingo Molnar <mingo@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>
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On Sat, Apr 29, 2017 at 7:18 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Dan Williams <dan.j.williams@intel.com> wrote:
>
>> Kirill points out that the calls to {get,put}_dev_pagemap() can be
>> removed from the mm fast path if we take a single get_dev_pagemap()
>> reference to signify that the page is alive and use the final put of the
>> page to drop that reference.
>>
>> This does require some care to make sure that any waits for the
>> percpu_ref to drop to zero occur *after* devm_memremap_page_release(),
>> since it now maintains its own elevated reference.
>>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Jérôme Glisse <jglisse@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
>> Suggested-by: Kirill Shutemov <kirill.shutemov@linux.intel.com>
>> Tested-by: Kirill Shutemov <kirill.shutemov@linux.intel.com>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>
> This changelog is lacking an explanation about how this solves the crashes you
> were seeing.
>

Kirill? It wasn't clear to me why the conversion to generic
get_user_pages_fast() caused the reference counts to be off.