linux-kernel - Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device

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Message-ID: <CAPcyv4jCfMwthPwbE-iuvef1KkMYUtA=qAydgfJzH0_otXoAOg@mail.gmail.com>
Date:   Fri, 28 Apr 2017 10:41:22 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()
 with a single reference

On Fri, Apr 28, 2017 at 10:34 AM, Jerome Glisse <jglisse@...hat.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.
>
> This is NAK from HMM point of view as i need those call. So if you remove
> them now i will need to add them back as part of HMM.

I thought you only need them at page free time? You can still hook __put_page().