Re: [PATCH 45 of 66] remove PG_buddy

[Andrea Arcangeli]

On Thu, Nov 18, 2010 at 04:08:01PM +0000, Mel Gorman wrote:
> On Wed, Nov 03, 2010 at 04:28:20PM +0100, Andrea Arcangeli wrote:
> > From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > 
> > PG_buddy can be converted to _mapcount == -2. So the PG_compound_lock can be
> > added to page->flags without overflowing (because of the sparse section bits
> > increasing) with CONFIG_X86_PAE=y and CONFIG_X86_PAT=y. This also has to move
> > the memory hotplug code from _mapcount to lru.next to avoid any risk of
> > clashes. We can't use lru.next for PG_buddy removal, but memory hotplug can use
> > lru.next even more easily than the mapcount instead.
> > 
> 
> Does this make much of a difference? I confess I didn't read the patch closely
> because I didn't get the motivation.

The motivation is described in the first line. If I wouldn't remove
PG_buddy, introducing PG_compound_lock would overflow the 32bit build
with CONFIG_X86_PAE=y and CONFIG_X86_PAT=y. The bitflag easier to nuke
was PG_buddy.
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