Re: [patch 0/4] mm: de-skew page refcount

[Linus Torvalds <torvalds@osdl.org>]

On Thu, 19 Jan 2006, Nick Piggin wrote:
> 
> But actually it doesn't matter that we might touch page_count, only
> that we not clear PageLRU. So the enabler is simply moving the
> TestClearPageLRU after the get_page_testone.

One side note on your patch: the pure bit _test_ operation is very cheap, 
but the "bit change" operation is very expensive (and not really any less 
expensive than the "test-and-change" one).

So the patch to avoid "test_and_clear_bit()" really helps only if the test 
usually results in us not doing the clear. Is that the case? Hmm..

So I _think_ that at least the case in "isolate_lru_page()", you'd 
actually be better off doing the "test-and-clear" instead of separate 
"test" and "clear-bit" ops, no? In that one, it would seem that 99+% of 
the time, the bit is set (because we tested it just before getting the 
lock).

No?

> I needed the de-skewing patch for something unrelated and it seemed that 
> it opened the possibility for the following optimisations (ie. because 
> we no longer touch a page after its refcount goes to zero).
>
> But actually it doesn't matter that we might touch page_count, only
> that we not clear PageLRU. So the enabler is simply moving the
> TestClearPageLRU after the get_page_testone.

Yes.

Now, that whole "we might touch the page count" thing does actually worry 
me a bit. The locking rules are subtle (but they -seem- safe: before we 
actually really put the page on the free-list in the freeing path, we'll 
have locked the LRU list if it was on one).

But if you were to change _that_ one to a

	atomic_add_unless(&page->counter, 1, -1);

I think that would be a real cleanup. And at that point I won't even 
complain that "atomic_inc_test()" is faster - that "get_page_testone()" 
thing is just fundamentally a bit scary, so I'd applaud it regardless.

(The difference: the "counter skewing" may be unexpected, but it's just a 
simple trick. In contrast, the "touch the count after the page may be 
already in the freeing stage" is a scary subtle thing. Even if I can't 
see any actual bug in it, it just worries me in a way that offsetting a 
counter by one does not..)

		Linus
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