Re: page fault fastpath patch v2: fix race conditions, stats for 8,32 and 512 cpu SMP

[William Lee Irwin III]

On Tue, Aug 17, 2004 at 08:28:44AM -0700, Christoph Lameter wrote:
> This is the second release of the page fault fastpath path. The fast path
> avoids locking during the creation of page table entries for anonymous
> memory in a threaded application running on a SMP system. The performance
> increases significantly for more than 4 threads running concurrently.
> Changes:
> - Insure that it is safe to call the various functions without holding
> the page_table_lock.
> - Fix cases in rmap.c where a pte could be cleared for a very short time
> before being set to another value by introducing a pte_xchg function. This
> created a potential race condition with the fastpath code which checks for
> a cleared pte without holding the page_table_lock.
> - i386 support
> - Various cleanups
> Issue remaining:
> - The fastpath increments mm->rss without acquiring the page_table_lock.
> Introducing the page_table_lock even for a short time makes performance
> drop to the level before the patch.

Hmm. I'm suspicious but I can't immediately poke a hole in it as it
leaves most uses of ->page_table_lock in place. I can't help thinking
there's a more comprehensive attack on the locking in this area, either.

-- wli
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