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SubjectRe: [PATCH 9/10] mm/memcg: move lru_lock into lruvec
On Tue, 21 Feb 2012, Konstantin Khlebnikov wrote:
>
> On lumpy/compaction isolate you do:
>
> if (!PageLRU(page))
> continue
>
> __isolate_lru_page()
>
> page_relock_rcu_vec()
> rcu_read_lock()
> rcu_dereference()...
> spin_lock()...
> rcu_read_unlock()
>
> You protect page_relock_rcu_vec with switching pointers back to root.
>
> I do:
>
> catch_page_lru()
> rcu_read_lock()
> if (!PageLRU(page))
> return false
> rcu_dereference()...
> spin_lock()...
> rcu_read_unlock()
> if (PageLRU())
> return true
> if true
> __isolate_lru_page()
>
> I protect my catch_page_lruvec() with PageLRU() under single rcu-interval
> with locking.
> Thus my code is better, because it not requires switching pointers back to
> root memcg.

That sounds much better, yes - if it does work reliably.

I'll have to come back to think about your locking later too;
or maybe that's exactly where I need to look, when investigating
the mm_inline.h:41 BUG.

But at first sight, I have to say I'm very suspicious: I've never found
PageLRU a good enough test for whether we need such a lock, because of
races with those pages on percpu lruvec about to be put on the lru.

But maybe once I look closer, I'll find that's handled by your changes
away from lruvec; though I'd have thought the same issue exists,
independent of whether the pending pages are in vector or list.

Hugh

>
> Meanwhile after seeing your patches, I realized that this rcu-protection is
> required only for lock-by-pfn in lumpy/compaction isolation.
> Thus my locking should be simplified and optimized.


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