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

[Dan Williams]

On Fri, Sep 28, 2018 at 1:45 AM Oscar Salvador
<osalvador@xxxxxxxxxxxxxxxxxx> 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.