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Date: Tue, 30 Mar 2010 11:35:01 -0500 (CDT)
From: Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH 36 of 41] remove PG_buddy
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To: Andrea Arcangeli <aarcange@redhat.com>
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On Tue, 30 Mar 2010, Andrea Arcangeli wrote:

> > Looks like SLUB also uses _mapcount for some fun purposes:
> >
> >         struct page {
> >                 unsigned long flags;            /* Atomic flags, some possibly
> >                                                  * updated asynchronously */
> >                 atomic_t _count;                /* Usage count, see below. */
> >                 union {
> >                         atomic_t _mapcount;     /* Count of ptes mapped in mms,
> >                                                  * to show when page is mapped
> >                                                  * & limit reverse map searches.
> >                                                  */
> >                         struct {                /* SLUB */
> >                                 u16 inuse;
> >                                 u16 objects;
> >                         };
> >                 };
> >
> > I guess those don't *really* become a problem in practice until we get a
> > really large page size that can hold >=64k objects.  But, at that point,
> > we're overflowing the types anyway (or really close to it).
>
> Maybe we should add a BUG_ON in slub in case anybody runs this on
> PAGE_SIZE == 2M (to avoid silent corruption).

SLUB has been verified a long time ago to run fine with 2M pages.

Just specify

slub_min_order=9

on the kernel command line to get a system booted up with 2M slab pages.

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