Re: [PATCH 36 of 41] remove PG_buddy

[Andrea Arcangeli <aarcange@redhat.com>]

On Tue, Mar 30, 2010 at 11:35:01AM -0500, Christoph Lameter wrote:
> 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.

sure no doubt if order 9 == 2M it's no problem, what I meant is:

#define PAGE_SIZE (4096*512)

2M >>> 64k so with order 0 == 2m I thought it might overflow, dunno!

As far as PG_buddy removal is concerned, if I use -2 I don't think
there can be any problem as it'd be next to overflow anyway like Dave
said.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>