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

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Message-ID: <20170430231421.GA15163@redhat.com>
Date:   Sun, 30 Apr 2017 19:14:24 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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 Sat, Apr 29, 2017 at 01:17:26PM +0300, Kirill A. Shutemov wrote:
> On Fri, Apr 28, 2017 at 03:33:07PM -0400, Jerome Glisse wrote:
> > On Fri, Apr 28, 2017 at 12:22:24PM -0700, Dan Williams wrote:
> > > Are you sure about needing to hook the 2 -> 1 transition? Could we
> > > change ZONE_DEVICE pages to not have an elevated reference count when
> > > they are created so you can keep the HMM references out of the mm hot
> > > path?
> > 
> > 100% sure on that :) I need to callback into driver for 2->1 transition
> > no way around that. If we change ZONE_DEVICE to not have an elevated
> > reference count that you need to make a lot more change to mm so that
> > ZONE_DEVICE is never use as fallback for memory allocation. Also need
> > to make change to be sure that ZONE_DEVICE page never endup in one of
> > the path that try to put them back on lru. There is a lot of place that
> > would need to be updated and it would be highly intrusive and add a
> > lot of special cases to other hot code path.
> 
> Could you explain more on where the requirement comes from or point me to
> where I can read about this.
> 

HMM ZONE_DEVICE pages are use like other pages (anonymous or file back page)
in _any_ vma. So i need to know when a page is freed ie either as result of
unmap, exit or migration or anything that would free the memory. For zone
device a page is free once its refcount reach 1 so i need to catch refcount
transition from 2->1

This is the only way i can inform the device that the page is now free. See

https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-v21&id=52da8fe1a088b87b5321319add79e43b8372ed7d

There is _no_ way around that.

Cheers,
Jérôme