linux-kernel - Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

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Date:   Tue, 9 Oct 2018 14:19:32 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     alexander.h.duyck@...ux.intel.com
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Pasha Tatashin <pavel.tatashin@...rosoft.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap

On Tue, Oct 9, 2018 at 1:34 PM Alexander Duyck
<alexander.h.duyck@...ux.intel.com> wrote:
>
> On 10/9/2018 11:04 AM, Dan Williams wrote:
> > On Tue, Oct 9, 2018 at 3:21 AM Yi Zhang <yi.z.zhang@...ux.intel.com> wrote:
[..]
> > That comment is incorrect, device-pages are never onlined. So I think
> > we can just skip that call to __SetPageReserved() unless the memory
> > range is MEMORY_DEVICE_{PRIVATE,PUBLIC}.
> >
>
> When pages are "onlined" via __free_pages_boot_core they clear the
> reserved bit, that is the reason for the comment. The reserved bit is
> meant to indicate that the page cannot be swapped out or moved based on
> the description of the bit.

...but ZONE_DEVICE pages are never onlined so I would expect
memmap_init_zone_device() to know that detail.

> I would think with that being the case we still probably need the call
> to __SetPageReserved to set the bit with the expectation that it will
> not be cleared for device-pages since the pages are not onlined.
> Removing the call to __SetPageReserved would probably introduce a number
> of regressions as there are multiple spots that use the reserved bit to
> determine if a page can be swapped out to disk, mapped as system memory,
> or migrated.

Right, this is what Yi is working on... the PageReserved flag is
problematic for KVM. Auditing those locations it seems as long as we
teach hibernation to avoid ZONE_DEVICE ranges we can safely not set
the reserved flag for DAX pages. What I'm trying to avoid is a local
KVM hack to check for DAX pages when the Reserved flag is not
otherwise needed.