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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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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