First of a batch of five patches to eliminate rmap's page_map_lock, replace
its trylocking by spinlocking, and use anon_vma to speed up swapoff.

Patches updated from the originals against 2.6.7-mm7: nothing new so I won't
spam the list, but including Manfred's SLAB_DESTROY_BY_RCU fixes, and omitting
the unuse_process mmap_sem fix already in 2.6.8-rc3.


This patch:

Replace the PG_anon page->flags bit by setting the lower bit of the pointer in
page->mapping when it's anon_vma: PAGE_MAPPING_ANON bit.

We're about to eliminate the locking which kept the flags and mapping in
synch: it's much easier to work on a local copy of page->mapping, than worry
about whether flags and mapping are in synch (though I imagine it could be
done, at greater cost, with some barriers).
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>