Re: [PATCHv3 1/2] mm: introduce vm_ops->map_pages()

[Andrew Morton]

On Thu, 27 Feb 2014 13:59:59 -0800 Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:

> On 02/27/2014 11:53 AM, Kirill A. Shutemov wrote:
> > +#define FAULT_AROUND_ORDER 4
> > +#define FAULT_AROUND_PAGES (1UL << FAULT_AROUND_ORDER)
> > +#define FAULT_AROUND_MASK ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1)
> 
> Looking at the performance data made me think of this: do we really want
> this to be static?  It seems like the kind of thing that will cause a
> regression _somewhere_.

Yes, allowing people to tweak it at runtime would improve testability a
lot.

I don't think we want to let yet another tunable out into the wild
unless we really need to - perhaps a not-for-mainline add-on patch, or
something in debugfs so we have the option of taking it away later.

> Also, the folks with larger base bage sizes probably don't want a
> FAULT_AROUND_ORDER=4.  That's 1MB of fault-around for ppc64, for example.

Yup, we don't want the same app to trigger dramatically different
kernel behaviour when it is moved from x86 to ppc.
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