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authorNick Piggin <npiggin@suse.de>2008-07-25 19:45:30 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-07-26 12:00:06 -0700
commite286781d5f2e9c846e012a39653a166e9d31777d (patch)
tree14958fe6d8f3e0459c96c68b3034ea2433ab85ac /drivers
parent47feff2c8eefe85099f87c43d3096855f0085ca0 (diff)
mm: speculative page references
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: Nick 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: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Signed-off-by: Hugh Dickins <hugh@veritas.com> Acked-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/net/cassini.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/drivers/net/cassini.c b/drivers/net/cassini.c
index 83768df2780..f1936d51b45 100644
--- a/drivers/net/cassini.c
+++ b/drivers/net/cassini.c
@@ -576,6 +576,18 @@ static void cas_spare_recover(struct cas *cp, const gfp_t flags)
list_for_each_safe(elem, tmp, &list) {
cas_page_t *page = list_entry(elem, cas_page_t, list);
+ /*
+ * With the lockless pagecache, cassini buffering scheme gets
+ * slightly less accurate: we might find that a page has an
+ * elevated reference count here, due to a speculative ref,
+ * and skip it as in-use. Ideally we would be able to reclaim
+ * it. However this would be such a rare case, it doesn't
+ * matter too much as we should pick it up the next time round.
+ *
+ * Importantly, if we find that the page has a refcount of 1
+ * here (our refcount), then we know it is definitely not inuse
+ * so we can reuse it.
+ */
if (page_count(page->buffer) > 1)
continue;