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Date:   Tue, 12 Mar 2019 04:39:36 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Roman Gushchin <guro@fb.com>
cc:     "Tobin C. Harding" <tobin@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pekka Enberg <penberg@cs.helsinki.fi>,
        Matthew Wilcox <willy@infradead.org>,
        Tycho Andersen <tycho@tycho.ws>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 04/15] slub: Enable Slab Movable Objects (SMO)
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On Mon, 11 Mar 2019, Roman Gushchin wrote:

> > +static inline void *alloc_scratch(struct kmem_cache *s)
> > +{
> > +	unsigned int size = oo_objects(s->max);
> > +
> > +	return kmalloc(size * sizeof(void *) +
> > +		       BITS_TO_LONGS(size) * sizeof(unsigned long),
> > +		       GFP_KERNEL);
>
> I wonder how big this allocation can be?
> Given that the reason for migration is probably highly fragmented memory,
> we probably don't want to have a high-order allocation here. So maybe
> kvmalloc()?

The smallest object size is 8 bytes which is one word which would be
places in an order 0 page. So it comes out to about a page again.

Larger allocation orders are possible if the slab pages itself can have
larger orders of course. If you set the min_order to the huge page order
then we can have similar sized orders for the allocation of the scratch
space. However, that is not a problem since the allocations for the slab
pages itself are also already of that same order.