From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4317F071.1070403@yahoo.com.au> Date: Fri, 02 Sep 2005 16:25:53 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: New lockless pagecache Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Linux Memory Management , linux-kernel List-ID: There were a number of problems with the old lockless pagecache: - page_count of free pages was unstable, which meant an extra atomic operation when allocating a new page (had to use get_page instead of set_page_count). - This meant a new page flag PG_free had to be introduced. - Also needed a spin loop and memory barriers added to the page allocator to prevent a free page with a speculative reference on it from being allocated. - This introduced the requirement that interrupts be disabled in page_cache_get_speculative to prevent deadlock, which was very disheartening. - Too complex. - To cap it off, there was an "unsolvable" race whereby a second page_cache_get_speculative would not be able to detect it had picked up a free page, and try to free it again. The elegant solution was right under my nose the whole time. I introduced an atomic_cmpxchg and use that to ensure we don't touch ->_count of a free page. This solves all the above problems. I think this is getting pretty stable. No guarantees of course, but it would be great if anyone gave it a test. -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org