linux-kernel - Re: [PATCH] SLOB allocator imcompatible SLAB

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Message-ID: <20070626045635.GC11115@waste.org>
Date:	Mon, 25 Jun 2007 23:56:35 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] SLOB allocator imcompatible SLAB

On Tue, Jun 26, 2007 at 02:06:15PM +1000, Nick Piggin wrote:
> Yoshinori Sato wrote:
> >At Fri, 22 Jun 2007 09:56:35 -0500,
> >Matt Mackall wrote:
> >
> >>On Fri, Jun 22, 2007 at 05:08:07PM +0900, Yoshinori Sato wrote:
> >>
> >>>Because the page which SLOB allocator got does not have PG_slab,
> >>
> >>This is for a NOMMU system?
> >
> >
> >Yes.
> > 
> >
> >>You're using an old kernel with an old version of SLOB. SLOB in newer
> >>kernels actually sets per-page flags. Nick, can you see any reason not
> >>to s/PG_active/PG_slab/ in the current code?
> 
> The problem with this is that PG_private is used only for the SLOB
> part of the allocator and not the bigblock part.

That's fine, at least for the purposes of kobjsize. We only mark
actual SLOB-managed pages, kobjsize assumes the rest are alloc_pages
and that's indeed what they are.

> We _could_ just bite the bullet and have SLOB set PG_slab, however
> that would encouarage more users of this flag which we should hope
> to get rid of one day.
> 
> The real problem is that nommu wants to get the size of either
> kmalloc or alloc_pages objects and it needs to differentiate
> between them. So I would rather nommu to take its own page flag
> (could overload PG_swapcache, perhaps?), and set that flag on
> pages it allocates directly, then uses that to determine whether
> to call ksize or not.

I think we already established on the last go-round that the kobjsize
scheme was rather hopelessly broken anyway. 

-- 
Mathematics is the supreme nostalgia of our time.
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