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SubjectRe: [PATCH 9/10] mm/memcg: move lru_lock into lruvec
Hugh Dickins wrote:
> 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.

pages_count[] updates looks correct.
This really may be bug in locking, and this VM_BUG_ON catch it before list-debug.

>
> 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.

Are you talking about my per-cpu page-lists for lru-adding?
This is just an unnecessary patch, I don't know why I include it into v2 set.
It does not protect anything.

>
> 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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