linux-kernel - Re: [PATCH] mm: Optimise nth

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Message-ID: <20210414185157.GU2531743@casper.infradead.org>
Date:   Wed, 14 Apr 2021 19:51:57 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
        Douglas Gilbert <dougg@...que.net>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] mm: Optimise nth_page for contiguous memmap

On Wed, Apr 14, 2021 at 05:24:42PM +0200, David Hildenbrand wrote:
> On 13.04.21 21:46, Matthew Wilcox (Oracle) wrote:
> > +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> >   #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
> > +#else
> > +#define nth_page(page,n) ((page) + (n))
> > +#endif
> 
> For sparsemem we could optimize within a single memory section. But not sure
> if it's worth the trouble.

Not only is it not worth the trouble, I suspect it's more expensive to
test-and-branch than just unconditionally call pfn_to_page() and
page_to_pfn().  That said, I haven't measured.

SPARSEMEM_VMEMMAP is default Y, and enabled by arm64, ia64, powerpc,
riscv, s390, sparc and x86.  I mean ... do we care any more?

> Reviewed-by: David Hildenbrand <david@...hat.com>
> 
> -- 
> Thanks,
> 
> David / dhildenb
>