lockless pagecache followups

lockless pagecache followups

[Nick Piggin]

Hi,

I'm going to submit my lockless pagecache patch to -mm. So I should
clarify a few questions I didn't have time or thought capacity to
answer in my OLS presentation when they were asked:

- The numbers in the presentation were with !CONFIG_PREEMPT kernels.

- I believe the speculative reference retry mechanism *could* use a
   seqlock rather than PG_nonewrefs + pagecache recheck. So that was a
   good question :) However, this doesn't fix the "all pages have an
   unstable refcount" problem.

   Also, it means readers will contend cachelines with writers in
   different parts of the file, and does make a finer grained write
   side possibly more difficult in some parts. So I prefer the
   custom locking protocol.

- The flush_dcache_mmap_lock AFAIKS(?) is logically a different lock
   from the pagecache tree_lock. I guess it just uses tree_lock
   because it can. So that path should be sped up with the rwlock
   -> spinlock conversion, and reduced contention from find_get_page.

   If there are any problems in this area, I'd like to know what
   they are.

- Comments have been improved.

- I still can't see how the lockless gang lookup could deadlock. If the
   radix-tree lookup finds 0 candidates, find_get_pages will return. The
   only time it retries is when the radix-tree lookup has found at least
   1 page, and the first page found has been moved.

- One question I would have liked asked is "why not use RCU for
   freeing the pages", although maybe that's obvious ;) It would solve
   the unstable refcount, and the atomic_inc_not_zero problems, however
   I think RCU would be too much burden on the pagecache freeing side.

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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