Traditionally we've not flushed TLB after clearing the young/referenced bit,
it has seemed just a waste of time.  Russell King points out that on some
architectures, with the move from 2.4 mm sweeping to 2.6 rmap, this may be a
serious omission: very frequently referenced pages never re-marked young, and
the worst choices made for unmapping.

So, replace ptep_test_and_clear_young by ptep_clear_flush_young throughout
rmap.c.  Originally I'd imagined making some kind of TLB gather optimization,
but don't see what now: whether worth it rather depends on how common
cross-cpu flushes are, and whether global or not.

ppc and ppc64 have already found this issue, and worked around it by arranging
TLB flush from their ptep_test_and_clear_young: with the aid of pgtable rmap
pointers.  I'm hoping ptep_clear_flush_young will allow ppc and ppc64 to
remove that special code, but won't change them myself.

It's worth noting that it is Andrea's anon_vma rmap which makes the vma
available for ptep_clear_flush_young in page_referenced_one: anonmm and
pte_chains would both need an additional find_vma for that.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>