linux-kernel - Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

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Date:   Thu, 12 Jul 2018 12:26:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.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>,
        "Wangkai (Kevin C)" <wangkai86@...wei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

On 07/12/2018 12:04 PM, James Bottomley wrote:
> On Thu, 2018-07-12 at 11:54 -0400, Waiman Long wrote:
>>
>> It is not that dentry cache is harder to get rid of than the other
>> memory. It is that the ability of generate unlimited number of
>> negative dentries that will displace other useful memory from the
>> system. What the patch is trying to do is to have a warning or
>> notification system in place to spot unusual activities in regard to
>> the number of negative dentries in the system. The system
>> administrators can then decide on what to do next.
> But every cache has this property: I can cause the same effect by doing
> a streaming read on a multi gigabyte file: the page cache will fill
> with the clean pages belonging to the file until I run out of memory
> and it has to start evicting older cache entries.  Once we hit the
> steady state of minimal free memory, the mm subsytem tries to balance
> the cache requests (like my streaming read) against the existing pool
> of cached objects.
>
> The question I'm trying to get an answer to is why does the dentry
> cache need special limits when the mm handling of the page cache (and
> other mm caches) just works?
>
> James
>

I/O activities can be easily tracked. Generation of negative dentries,
however, is more insidious. So the ability to track and be notified when
too many negative dentries are created can be a useful tool for the
system administrators. Besides, there are paranoid users out there who
want to have control of as much as system parameters as possible.

Cheers,
Longman