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

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Date:   Fri, 20 Aug 2021 07:05:04 +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 Tue, Aug 17, 2021 at 11:44:54AM -0400, Felix Kuehling wrote:
> >> 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?

The old DEVICE_PUBLIC mostly different in that it allowed the page
to be returned from vm_normal_page, which I think was horribly buggy.

But the point is not to bring back these old semantics.  The idea
is to be able to differeniate between your new coherent on-device
memory and the existing DEVICE_GENERIC.  That is call the
code in free_devmap_managed_page that is currently only used
for device private pages also for your new public device pages without
affecting the devdax and xen use cases.