Re: [PATCH] mm: Optimise nth_page for contiguous memmap

[Matthew Wilcox]

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@xxxxxxxxxx>
> 
> -- 
> Thanks,
> 
> David / dhildenb
>