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Date:   Mon, 11 Jul 2022 15:00:12 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Alex Sierra <alex.sierra@amd.com>, jgg@nvidia.com,
        Felix.Kuehling@amd.com, linux-mm@kvack.org, rcampbell@nvidia.com,
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        akpm@linux-foundation.org
Subject: Re: [PATCH v8 07/15] mm/gup: migrate device coherent pages when
 pinning instead of failing
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On Mon, Jul 11, 2022 at 03:35:42PM +0200, David Hildenbrand wrote:
> > +		/*
> > +		 * Device coherent pages are managed by a driver and should not
> > +		 * be pinned indefinitely as it prevents the driver moving the
> > +		 * page. So when trying to pin with FOLL_LONGTERM instead try
> > +		 * to migrate the page out of device memory.
> > +		 */
> > +		if (folio_is_device_coherent(folio)) {
> > +			WARN_ON_ONCE(PageCompound(&folio->page));
> 
> Maybe that belongs into migrate_device_page()?

... and should be simply WARN_ON_ONCE(folio_test_large(folio)).
No need to go converting it back into a page in order to test things
that can't possibly be true.