linux-kernel - Re: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries

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Message-ID: <0cd48ec2-fd71-7c3c-fcf4-82df9cacd1f6@redhat.com>
Date:   Mon, 21 Aug 2017 09:34:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Wangkai (Kevin,C)" <wangkai86@...wei.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "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>
Subject: Re: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries

On 08/20/2017 11:23 PM, Wangkai (Kevin,C) wrote:
>
> Yes, I have add some trace info for the dentry state changed, with dentry flag and reference count:
>
> File create:
> [   42.636675] dentry [xxxx_1234] 0xffff880230be8180 flag 0x0 ref 1 ev dentry alloc
> File close:
> [   42.637421] dentry [xxxx_1234] 0xffff880230be8180 flag 0x4800c0 ref 0 ev dput called
>
> Unlink lookup:
> [  244.658086] dentry [xxxx_1234] 0xffff880230be8180 flag 0x4800c0 ref 1 ev d_lookup
> Unlink d_delete:
> [  244.658254] dentry [xxxx_1234] 0xffff880230be8180 flag 0x800c0 ref 1 ev d_lockref ref 1
> Unlink dput:
> [  244.658438] dentry [xxxx_1234] 0xffff880230be8180 flag 0x800c0 ref 0 ev dput called
>
> The end, dentry's flag stay at 0x800c0, but this dentry was not freed, keeped by the dcache as unused,
> After tens of thousands of the dentries slow down the dentry lookup performance, kernel memory usage
> Keep high.
>
> Regards,
> Kevin

That is expected. The kernel does not get rid of negative dentries until
the shrinker is called because of memory pressure. Negative dentries do
help to improve file lookup performance. However, too much of negative
dentries suppress the amount of free memory available for other use.
That is why I send out my patch to limit the number of negative dentries
outstanding.

Cheers,
Longman