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Subject: Re: [PATCH v6 08/13] mm: call pgmap->ops->page_free for
 DEVICE_GENERIC pages
To:     Christoph Hellwig <hch@lst.de>
Cc:     Alex Sierra <alex.sierra@amd.com>, akpm@linux-foundation.org,
        linux-mm@kvack.org, rcampbell@nvidia.com,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
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        jgg@nvidia.com, jglisse@redhat.com,
        Roger Pau Monne <roger.pau@citrix.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
References: <20210813063150.2938-1-alex.sierra@amd.com>
 <20210813063150.2938-9-alex.sierra@amd.com> <20210815154047.GC32384@lst.de>
 <7a55366f-bd65-7ab9-be9e-3bfd3aea3ea1@amd.com> <20210817055031.GC4895@lst.de>
From:   Felix Kuehling <felix.kuehling@amd.com>
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Date:   Tue, 17 Aug 2021 11:44:54 -0400
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Am 2021-08-17 um 1:50 a.m. schrieb Christoph Hellwig:
> On Mon, Aug 16, 2021 at 03:00:49PM -0400, Felix Kuehling wrote:
>> Am 2021-08-15 um 11:40 a.m. schrieb Christoph Hellwig:
>>> On Fri, Aug 13, 2021 at 01:31:45AM -0500, Alex Sierra wrote:
>>>> Add MEMORY_DEVICE_GENERIC case to free_zone_device_page callback.
>>>> Device generic type memory case is now able to free its pages properly.
>>> How is this going to work for the two existing MEMORY_DEVICE_GENERIC
>>> that now change behavior?  And which don't have a ->page_free callback
>>> at all?
>> That's a good catch. Existing drivers shouldn't need a page_free
>> callback if they didn't have one before. That means we need to add a
>> NULL-pointer check in free_device_page.
> Also the other state clearing (__ClearPageWaiters/mem_cgroup_uncharge/
> ->mapping = NULL).
>
> In many ways this seems like you want to bring back the DEVICE_PUBLIC
> pgmap type that was removed a while ago due to the lack of users
> instead of overloading the generic type.

I think so. I'm not clear about how DEVICE_PUBLIC differed from what
DEVICE_GENERIC is today. As I understand it, DEVICE_PUBLIC was removed
because it was unused and also known to be broken in some ways.
DEVICE_GENERIC seemed close enough to what we need, other than not being
supported in the migration helpers.

Would you see benefit in re-introducing DEVICE_PUBLIC as a distinct
memory type from DEVICE_GENERIC? What would be the benefits of making
that distinction?

Thanks,
  Felix