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:   Fri, 28 Sep 2018 08:50:46 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     osalvador@...hadventures.net
Cc:     Michal Hocko <mhocko@...nel.org>,
        alexander.h.duyck@...ux.intel.com, 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>,
        Dave Jiang <dave.jiang@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Logan Gunthorpe <logang@...tatee.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 Fri, Sep 28, 2018 at 1:45 AM Oscar Salvador
<osalvador@...hadventures.net> wrote:
>
> On Fri, Sep 28, 2018 at 10:12:24AM +0200, Oscar Salvador wrote:
> > Although I am not sure about leaving memmap_init_zone unprotected.
> > For the normal memory, that is not a problem since the memblock's lock
> > protects us from touching the same pages at the same time in online/offline_pages,
> > but for HMM/devm the story is different.
> >
> > I am totally unaware of HMM/devm, so I am not sure if its protected somehow.
> > e.g: what happens if devm_memremap_pages and devm_memremap_pages_release are running
> > at the same time for the same memory-range (with the assumption that the hotplug-lock
> > does not protect move_pfn_range_to_zone anymore).
>
> I guess that this could not happen since the device is not linked to devm_memremap_pages_release
> until the end with:
>
> devm_add_action(dev, devm_memremap_pages_release, pgmap)

It's a bug if devm_memremap_pages and devm_memremap_pages_release are
running simultaneously for the same range. This is enforced by the
requirement that the caller has done a successful request_region() on
the range before the call to map pages.