linux-kernel - Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private

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Message-ID: <e384a09a-23a2-6a9b-dda6-db93e26c8f66@nvidia.com>
Date:   Fri, 6 Nov 2020 12:34:49 -0800
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Jerome Glisse" <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Alistair Popple" <apopple@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Zi Yan <ziy@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private_page()


On 11/6/20 4:14 AM, Matthew Wilcox wrote:
> 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.

I'm not sure what the simplification is.
If you mean the name change from prep_transhuge_page() to thp_prep(),
that seems fine to me. The following could also be renamed to
thp_prep_device_private_page() or similar.

>> +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 
> 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.

I did see those changes. It might help some device drivers to do DMA in
larger than PAGE_SIZE blocks but less than PMD_SIZE. It might help
reduce page table sizes since 2MB, 64K, and 4K are commonly supported
GPU page sizes. The MIGRATE_PFN_COMPOUND flag is intended to indicate
that the page size is determined by page_size() so I was thinking ahead
to other than PMD sized pages. However, when migrating a pte_none() or
pmd_none() page, there is no source page to determine the size.
Maybe I need to encode the page order in the migrate PFN entry like
hmm_range_fault().

Anyway, I agree that thinking about page sizes other than PMD is good.