linux-kernel - Re: [patch 2/2] fs: buffer: move allocation failure loop into the allocator

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Date:	Wed, 4 Dec 2013 15:17:08 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, azurIt <azurit@...ox.sk>,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Christian Casteyde <casteyde.christian@...e.fr>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [patch 2/2] fs: buffer: move allocation failure loop into the
 allocator

On Tue, 3 Dec 2013, Andrew Morton wrote:

> >  	page = alloc_slab_page(alloc_gfp, node, oo);
> >  	if (unlikely(!page)) {
> >  		oo = s->min;
>
> What is the value of s->min?  Please tell me it's zero.

It usually is.

> > @@ -1349,7 +1350,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> >  		&& !(s->flags & (SLAB_NOTRACK | DEBUG_DEFAULT_FLAGS))) {
> >  		int pages = 1 << oo_order(oo);
> >
> > -		kmemcheck_alloc_shadow(page, oo_order(oo), flags, node);
> > +		kmemcheck_alloc_shadow(page, oo_order(oo), alloc_gfp, node);
>
> That seems reasonable, assuming kmemcheck can handle the allocation
> failure.
>
>
> Still I dislike this practice of using unnecessarily large allocations.
> What does it gain us?  Slightly improved object packing density.
> Anything else?

The fastpath for slub works only within the bounds of a single slab page.
Therefore a larger frame increases the number of allocation possible from
the fastpath without having to use the slowpath and also reduces the
management overhead in the partial lists.

There is a kernel parameter that can be used to control the maximum order

	slub_max_order

The default is PAGE_ALLOC_COSTLY_ORDER. See also
Documentation/vm/slub.txt.

Booting with slub_max_order=1 will force order 0/1 pages.

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