linux-kernel - Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed

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Date:   Thu, 30 Aug 2018 17:48:50 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        "Wangkai (Kevin C)" <wangkai86@...wei.com>
Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be
 reclaimed

On 08/30/2018 03:20 AM, Michal Hocko wrote:
> On Wed 29-08-18 15:58:52, Waiman Long wrote:
>> On 08/29/2018 03:51 AM, Michal Hocko wrote:
>>> On Tue 28-08-18 13:19:40, Waiman Long wrote:
>>>> For negative dentries that are accessed once and never used again, they
>>>> should be removed first before other dentries when shrinker is running.
>>>> This is done by putting negative dentries at the head of the LRU list
>>>> instead at the tail.
>>>>
>>>> A new DCACHE_NEW_NEGATIVE flag is now added to a negative dentry when it
>>>> is initially created. When such a dentry is added to the LRU, it will be
>>>> added to the head so that it will be the first to go when a shrinker is
>>>> running if it is never accessed again (DCACHE_REFERENCED bit not set).
>>>> The flag is cleared after the LRU list addition.
>>> Placing object to the head of the LRU list can be really tricky as Dave
>>> pointed out. I am not familiar with the dentry cache reclaim so my
>>> comparison below might not apply. Let me try anyway.
>>>
>>> Negative dentries sound very similar to MADV_FREE pages from the reclaim
>>> POV. They are primary candidate for reclaim, yet you want to preserve
>>> aging to other easily reclaimable objects (including other MADV_FREE
>>> pages). What we do for those pages is to move them from the anonymous
>>> LRU list to the inactive file LRU list. Now you obviously do not have
>>> anon/file LRUs but something similar to active/inactive LRU lists might
>>> be a reasonably good match. Have easily reclaimable dentries on the
>>> inactive list including negative dentries. If negative entries are
>>> heavily used then they can promote to the active list because there is
>>> no reason to reclaim them soon.
>>>
>>> Just my 2c
>> As mentioned in my reply to Dave, I did considered using a 2 LRU list
>> solution. However, that will add more complexity to the dcache LRU
>> management code than my current approach and probably more potential for
>> slowdown.
> I completely agree with Dave here. This is not easy but trying to sneak
> in something that works for an _artificial_ workload is simply a no go.
> So if it takes to come with a more complex solution to cover more
> general workloads then be it. Someone has to bite a bullet and explore
> that direction. It won't be a simple project but well, if negative
> dentries really matter then it is worth making the reclaim design robust
> and comprehensible rather than adhoc and unpredictable.

OK, I will need to spend more time to think about a better way of doing
that.

Cheers,
Longman