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SubjectRe: [PATCH v3 3/7] mm/lru: replace pgdat lru_lock with lruvec lock
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在 2019/11/19 上午12:11, Johannes Weiner 写道:
> On Sat, Nov 16, 2019 at 11:15:02AM +0800, Alex Shi wrote:
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 62470325f9bc..cf274739e619 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1246,6 +1246,42 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
>> return lruvec;
>> }
>>
>> +struct lruvec *lock_page_lruvec_irq(struct page *page,
>> + struct pglist_data *pgdat)
>> +{
>> + struct lruvec *lruvec;
>> +
>> +again:
>> + lruvec = mem_cgroup_page_lruvec(page, pgdat);
>> + spin_lock_irq(&lruvec->lru_lock);
>
> This isn't safe. Nothing prevents the page from being moved to a
> different memcg in between these two operations, and the lruvec having
> been freed by the time you try to acquire the spinlock.
>
> You need to use the rcu lock to dereference page->mem_cgroup and its
> lruvec when coming from the page like this.

Hi Johannes,

Yes, you are right. Thank a lot to point out this!
Could we use rcu lock to guard the lruvec till lruvec->lru_lock getten?

+struct lruvec *lock_page_lruvec_irq(struct page *page,
+ struct pglist_data *pgdat)
+{
+ struct lruvec *lruvec;
+
+again:
+ rcu_read_lock();
+ lruvec = mem_cgroup_page_lruvec(page, pgdat);
+ spin_lock_irq(&lruvec->lru_lock);
+ rcu_read_unlock();
+
+ /* lruvec may changed in commit_charge() */
+ if (lruvec != mem_cgroup_page_lruvec(page, pgdat)) {
+ spin_unlock_irq(&lruvec->lru_lock);
+ goto again;
+ }
+
+ return lruvec;
+}
>
> You also need to use page_memcg_rcu() to insert the appropriate
> lockless access barriers, which mem_cgroup_page_lruvec() does not do
> since it's designed for use with pgdat->lru_lock.
>

Since the page_memcg_rcu can not know it under a spin_lock, guess the following enhance is fine:
@@ -1225,7 +1224,7 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
goto out;
}

- memcg = page->mem_cgroup;
+ memcg = READ_ONCE(page->mem_cgroup);
/*


Millions thanks!
Alex
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