Re: [PATCH] mm: MADV_WILLNEED implementation for anonymous memory

From: Andrew Morton
Date: Thu Jan 31 2008 - 04:13:49 EST


On Thu, 31 Jan 2008 09:44:00 +0100 Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:

>
> On Wed, 2008-01-30 at 14:40 -0800, Andrew Morton wrote:
> > On Wed, 30 Jan 2008 18:28:59 +0100
> > Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> >
> > > Implement MADV_WILLNEED for anonymous pages by walking the page tables and
> > > starting asynchonous swap cache reads for all encountered swap pages.
> >
> > Why cannot this use (a perhaps suitably-modified) make_pages_present()?
>
> Because make_pages_present() relies on page faults to bring data in and
> will thus wait for all data to be present before returning.
>
> This solution is async; it will just issue a read for the requested
> pages and moves on.
>

I of course realise that. I also realise that swapin_readahead() is
_supposed_ to make the difference moot.

There's something you guys aren't telling us. Several things, actually.
Please don't do that.



Implementation-wise: make_pages_present() _can_ be converted to do this.
But it's a lot of patching, and the result will be a cleaner, faster and
smaller core MM. Whereas your approach is easy, but adds more code and
leaves the old stuff slow-and-dirty.

Guess which approach is preferred? ;)


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