• Nick Piggin's avatar
    mm: speculative page references · e286781d
    Nick Piggin authored
    If we can be sure that elevating the page_count on a pagecache page will
    pin it, we can speculatively run this operation, and subsequently check to
    see if we hit the right page rather than relying on holding a lock or
    otherwise pinning a reference to the page.
    
    This can be done if get_page/put_page behaves consistently throughout the
    whole tree (ie.  if we "get" the page after it has been used for something
    else, we must be able to free it with a put_page).
    
    Actually, there is a period where the count behaves differently: when the
    page is free or if it is a constituent page of a compound page.  We need
    an atomic_inc_not_zero operation to ensure we don't try to grab the page
    in either case.
    
    This patch introduces the core locking protocol to the pagecache (ie.
    adds page_cache_get_speculative, and tweaks some update-side code to make
    it work).
    
    Thanks to Hugh for pointing out an improvement to the algorithm setting
    page_count to zero when we have control of all references, in order to
    hold off speculative getters.
    
    [kamezawa.hiroyu@jp.fujitsu.com: fix migration_entry_wait()]
    [hugh@veritas.com: fix add_to_page_cache]
    [akpm@linux-foundation.org: repair a comment]
    Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
    Cc: Jeff Garzik <jeff@garzik.org>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Hugh Dickins <hugh@veritas.com>
    Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
    Reviewed-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: default avatarDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
    Acked-by: default avatarNick Piggin <npiggin@suse.de>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    e286781d
migrate.c 25.4 KB