linux-kernel - Re: [PATCH 00/10] foundations for reserve-based allocation

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Message-Id: <200708061140.05002.phillips@phunq.net>
Date:	Mon, 6 Aug 2007 11:40:04 -0700
From:	Daniel Phillips <phillips@...nq.net>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Phillips <phillips@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <clameter@....com>,
	Matt Mackall <mpm@...enic.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Steve Dickson <SteveD@...hat.com>
Subject: Re: [PATCH 00/10] foundations for reserve-based allocation

On Monday 06 August 2007 11:17, Peter Zijlstra wrote:
> lim_{n -> inf} (2^(n+1)/((2^n)+1)) = 2^lim_{n -> inf} ((n+1)-n) = 2^1 
= 2

Glad I asked :-)

> > Patch [3/10] adds a new field to struct page.
>
> No it doesn't.

True.  It is not immediately obvious from the declaration that the 
overloaded field is always freed up before anybody else needs to use 
the union.

> >   I do not think this is
> > necessary.   Allocating a page from reserve does not make it
> > special. All we care about is that the total number of pages taken
> > out of reserve is balanced by the total pages freed by a user of
> > the reserve.
>
> And how do we know a page was taken out of the reserves?
>
> This is done by looking at page->reserve (overload of page->index)
> and this value can be destroyed as soon as its observed. It is in a
> sense an extra return value.

Ah I see.  I used to let alloc_pages fail then repeat the allocation 
with __GFP_MEMALLOC set, which was easy but stupidly repetitious.  Your 
technique is better, though returning the status in the page still 
looks a little funny.  This is really about saving a page flag, no?

> > We do care about slab fragmentation in the sense that a slab page
> > may be pinned in the slab by an unprivileged allocation and so that
> > page may never be returned to the global page reserve.
>
> A slab page obtained from the reseserve will never serve an object to
> an unprivilidged allocation.
>
> >   One way to solve this is
> > to have a per slabpage flag indicating the page came from reserve,
> > and prevent mixing of privileged and unprivileged allocations on
> > such a page.
>
> is done.

Serves me right for not reading that bit.  So the score for that round 
is: peterz 3, phillips 0 ;-)

Regards,

Daniel
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