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From:   Zi Yan <ziy@nvidia.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     <linux-fsdevel@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH v3 01/25] mm: Introduce struct folio
Date:   Tue, 2 Mar 2021 12:47:20 -0500
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On 2 Mar 2021, at 8:22, Matthew Wilcox wrote:

> On Mon, Mar 01, 2021 at 03:26:11PM -0500, Zi Yan wrote:
>>> +static inline struct folio *next_folio(struct folio *folio)
>>> +{
>>> +	return folio + folio_nr_pages(folio);
>>
>> Are you planning to make hugetlb use folio too?
>>
>> If yes, this might not work if we have CONFIG_SPARSEMEM && !CONFIG_SPA=
RSEMEM_VMEMMAP
>> with a hugetlb folio > MAX_ORDER, because struct page might not be vir=
tually contiguous.
>> See the experiment I did in [1].
>
> Actually, how about proofing this against a future change?
>
> static inline struct folio *next_folio(struct folio *folio)
> {
> #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> 	pfn_t next_pfn =3D page_to_pfn(&folio->page) + folio_nr_pages(folio);
> 	return (struct folio *)pfn_to_page(next_pfn);
> #else
> 	return folio + folio_nr_pages(folio);
> #endif
> }
>
> (not compiled)

Yes, it should work. A better version might be that in the top half
you check folio order first and if the order >=3D MAX_ORDER, we use
the complicated code, otherwise just folio+folio_nr_pages(folio).

This CONFIG_SPARSEMEM && !CONFIG_SPARSEMEM_VMEMMAP is really not friendly=

to >=3DMAX_ORDER pages. Most likely I am going to make 1GB THP
rely on CONFIG_SPARSEMEM_VMEMMAP to avoid complicated code.

=E2=80=94
Best Regards,
Yan Zi

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