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

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Date:   Fri, 28 Apr 2017 14:00:06 -0400 (EDT)
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ingo Molnar <mingo@...nel.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().

No, i need a hook when page refcount reach 1, not 0. That being said
i don't care about put_dev_pagemap(page->pgmap); so that part of the
patch is fine from HMM point of view but i definitly need to hook my-
self in the general put_page() function.

So i will have to undo part of this patch for HMM (put_page() will
need to handle ZONE_DEVICE page differently).

Cheers,
Jérôme