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Date:   Fri, 6 Nov 2020 12:14:07 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     linux-mm@kvack.org, nouveau@lists.freedesktop.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Alistair Popple <apopple@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Bharata B Rao <bharata@linux.ibm.com>,
        Zi Yan <ziy@nvidia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Ben Skeggs <bskeggs@redhat.com>, Shuah Khan <shuah@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private_page()
Message-ID: <20201106121407.GQ17076@casper.infradead.org>
References: <20201106005147.20113-1-rcampbell@nvidia.com>
 <20201106005147.20113-2-rcampbell@nvidia.com>
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On Thu, Nov 05, 2020 at 04:51:42PM -0800, Ralph Campbell wrote:
> Add a helper function to allow device drivers to create device private
> transparent huge pages. This is intended to help support device private
> THP migrations.

I think you'd be better off with these calling conventions:

-void prep_transhuge_page(struct page *page)
+struct page *thp_prep(struct page *page)
 {
+       if (!page || compound_order(page) == 0)
+               return page;
        /*
-        * we use page->mapping and page->indexlru in second tail page
+        * we use page->mapping and page->index in second tail page
         * as list_head: assuming THP order >= 2
         */
+       BUG_ON(compound_order(page) == 1);
 
        INIT_LIST_HEAD(page_deferred_list(page));
        set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
+
+       return page;
 }

It simplifies the users.

> +void prep_transhuge_device_private_page(struct page *page)
> +{
> +	prep_compound_page(page, HPAGE_PMD_ORDER);
> +	prep_transhuge_page(page);
> +	/* Only the head page has a reference to the pgmap. */
> +	percpu_ref_put_many(page->pgmap->ref, HPAGE_PMD_NR - 1);
> +}
> +EXPORT_SYMBOL_GPL(prep_transhuge_device_private_page);

Something else that may interest you from my patch series is support
for page sizes other than PMD_SIZE.  I don't know what page sizes your
hardware supports.  There's no support for page sizes other than PMD
for anonymous memory, so this might not be too useful for you yet.