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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. [[email protected]: fix migration_entry_wait()] [[email protected]: fix add_to_page_cache] [[email protected]: repair a comment] Signed-off-by: Nick Piggin <[email protected]> Cc: Jeff Garzik <[email protected]> Cc: Benjamin Herrenschmidt <[email protected]> Cc: Paul Mackerras <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: "Paul E. McKenney" <[email protected]> Reviewed-by: Peter Zijlstra <[email protected]> Signed-off-by: Daisuke Nishimura <[email protected]> Signed-off-by: KAMEZAWA Hiroyuki <[email protected]> Signed-off-by: KOSAKI Motohiro <[email protected]> Signed-off-by: Hugh Dickins <[email protected]> Acked-by: Nick Piggin <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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