linux-ext4 - Re: [PATCH v6 08/13] mm: call pgmap->ops->page_free for DEVICE

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Date:   Tue, 17 Aug 2021 07:50:31 +0200
From:   Christoph Hellwig <hch@....de>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     Christoph Hellwig <hch@....de>, Alex Sierra <alex.sierra@....com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        rcampbell@...dia.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, jgg@...dia.com,
        jglisse@...hat.com, Roger Pau Monne <roger.pau@...rix.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v6 08/13] mm: call pgmap->ops->page_free for
 DEVICE_GENERIC pages

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.