Messages in this thread | | | Date | Fri, 02 Sep 2005 16:25:53 +1000 | From | Nick Piggin <> | Subject | New lockless pagecache |
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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.
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