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Date:   Wed, 8 Aug 2018 23:28:15 +0200
From:   Oscar Salvador <osalvador@techadventures.net>
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     Michal Hocko <mhocko@kernel.org>, akpm@linux-foundation.org,
        dan.j.williams@intel.com, pasha.tatashin@oracle.com,
        david@redhat.com, yasu.isimatu@gmail.com, logang@deltatee.com,
        dave.jiang@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Oscar Salvador <osalvador@suse.de>
Subject: Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and
 move it to offline_pages
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On Wed, Aug 08, 2018 at 12:58:15PM -0400, Jerome Glisse wrote:
> > If the former then I do not see any reason why we couldn't simply
> > refactor the code to expect a failure and drop the warning in that path.
> 
> Referring to newer case ie calling release_mem_region_adjustable() for
> ZONE_DEVICE too. It seems i got my recollection wrong in the sense that
> the resource is properly register as MEM but still we do not want to
> release it because the device driver might still be using the resource
> without struct page. The lifetime of the resource as memory with struct
> page and the lifetime of the resource as something use by the device
> driver are not tie together. The latter can outlive the former.
> 
> So when we hotremove ZONE_DEVICE we do not want to release the resource
> yet just to be on safe side and avoid some other driver/kernel component
> to decide to use that resource.

I checked the function that hot-removes the memory in HMM code.
hmm_devmem_pages_remove(), which gets called via hmm_devmem_remove(), is in charge
of hot-removing the memory.

Then, hmm_devmem_remove() will release the resource only if the resource is not of
type IORES_DESC_DEVICE_PUBLIC_MEMORY.

So I guess that there are cases(at least in HMM) where we release the resource when
hot-removing memory, but not always.

Looking at devm code, I could not see any place where we release the resource
when hot-removing memory.

So, if we are really left with such scenario, maybe the easiest way is to pass a parameter
from those paths to arch_remove_memory()->__remove_pages() to know
if we get called from device_functions, and so skip the call to release_mem_region_adjustable.

Thanks
-- 
Oscar Salvador
SUSE L3