|| ||Nick Piggin <firstname.lastname@example.org>|
|| ||Linux Memory Management <email@example.com>,
|| ||New lockless pagecache|
|| ||Fri, 02 Sep 2005 16:25:53 +1000|
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
- 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
- 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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