linux-kernel - Re: [PATCH RFC 2/3] mm: Add support for exposing if dev

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Date:   Tue, 4 Dec 2018 18:24:28 -0500
From:   Barret Rhoden <brho@...gle.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zhang Yi <yi.z.zhang@...ux.intel.com>,
        KVM list <kvm@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Dave Jiang <dave.jiang@...el.com>,
        "Zhang, Yu C" <yu.c.zhang@...el.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        David Hildenbrand <david@...hat.com>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>, rkrcmar@...hat.com,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap
 supports refcount pinning

Hi -

On 2018-12-04 at 14:51 Alexander Duyck
<alexander.h.duyck@...ux.intel.com> wrote:

[snip]

> > I think the confusion arises from the fact that there are a few MMIO
> > resources with a struct page and all the rest MMIO resources without.
> > The problem comes from the coarse definition of pfn_valid(), it may
> > return 'true' for things that are not System-RAM, because pfn_valid()
> > may be something as simplistic as a single "address < X" check. Then
> > PageReserved is a fallback to clarify the pfn_valid() result. The
> > typical case is that MMIO space is not caught up in this linear map
> > confusion. An MMIO address may or may not have an associated 'struct
> > page' and in most cases it does not.  
> 
> Okay. I think I understand this somewhat now. So the page might be
> physically there, but with the reserved bit it is not supposed to be
> touched.
> 
> My main concern with just dropping the bit is that we start seeing some
> other uses that I was not certain what the impact would be. For example
> the functions like kvm_set_pfn_accessed start going in and manipulating
> things that I am not sure should be messed with for a DAX page.

One thing regarding the accessed and dirty bits is that we might want
to have DAX pages marked dirty/accessed, even if we can't LRU-reclaim
or swap them.  I don't have a real example and I'm fairly ignorant
about the specifics here.  But one possibility would be using the A/D
bits to detect changes to a guest's memory for VM migration.  Maybe
there would be issues with KSM too.

Barret