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

[Nick Piggin <npiggin@suse.de>]

On Thu, Jan 19, 2006 at 08:36:14AM -0800, Linus Torvalds wrote:
> 
> 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?
> 

Well in isolate_lru_page, the test operation is actually optional
(ie. it is the conditional for a BUG). And I have plans for making
some of those configurable....

But at least on the G5, test_and_clear can be noticable (although
IIRC it was in the noise for _this_ articular case) because of the
memory barriers required.

> 
> 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).
> 

Yes, I think Andrew did his homework. I thought it through quite a bit
before sending the patches and again after your feedback. Subtle though,
no doubt.

> 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.
> 

Hmm... this is what the de-skew patch _did_ (although it was wrapped
in a function called get_page_unless_zero), in fact the main aim was
to prevent this twiddling and the de-skewing was just a nice side effect
(I guess the patch title is misleading).

So I'm confused...

> (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..)
> 

Don't worry, you'll be seeing that patch again -- it is required for
lockless pagecache.

Nick

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