linux-kernel - Re: [PATCHv3 1/2] mm: introduce vm_ops->map

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Date:	Thu, 27 Feb 2014 14:08:13 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>, Ning Qu <quning@...il.com>,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 1/2] mm: introduce vm_ops->map_pages()

On Thu, 27 Feb 2014 13:59:59 -0800 Dave Hansen <dave.hansen@...ux.intel.com> 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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