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Subject: Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock
From:   Alex Shi <alex.shi@linux.alibaba.com>
To:     Johannes Weiner <hannes@cmpxchg.org>, akpm@linux-foundation.org
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Date:   Thu, 16 Apr 2020 16:01:20 +0800
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在 2020/4/15 下午9:42, Alex Shi 写道:
> Hi Johannes,
> 
> Thanks a lot for point out!
> 
> Charging in __read_swap_cache_async would ask for 3 layers function arguments
> pass, that would be a bit ugly. Compare to this, could we move out the
> lru_cache add after commit_charge, like ksm copied pages?
> 
> That give a bit extra non lru list time, but the page just only be used only
> after add_anon_rmap setting. Could it cause troubles?

Hi Johannes & Andrew,

Doing lru_cache_add_anon during swapin_readahead can give a very short timing 
for possible page reclaiming for these few pages.

If we delay these few pages lru adding till after the vm_fault target page 
get memcg charging(mem_cgroup_commit_charge) and activate, we could skip the 
mem_cgroup_try_charge/commit_charge/cancel_charge process in __read_swap_cache_async().
But the cost is maximum SWAP_RA_ORDER_CEILING number pages on each cpu miss
page reclaiming in a short time. On the other hand, save the target vm_fault
page from reclaiming before activate it during that time.

Judging the lose and gain, and the example of shmem/ksm copied pages, I believe 
it's fine to delay lru list adding till activate the target swapin page.

Any comments are appreciated! 

Thanks
Alex


> 
> I tried to track down the reason of lru_cache_add here, but no explanation
> till first git kernel commit.
> 
> Thanks
> Alex 
>