linux-kernel - Re: [PATCH RFC 8/8] dcache: prevent flooding with negative dentries

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Date:   Fri, 8 May 2020 19:29:40 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH RFC 8/8] dcache: prevent flooding with negative dentries

On 08/05/2020 17.56, Matthew Wilcox wrote:
> On Fri, May 08, 2020 at 03:23:33PM +0300, Konstantin Khlebnikov wrote:
>> This patch implements heuristic which detects such scenarios and prevents
>> unbounded growth of completely unneeded negative dentries. It keeps up to
>> three latest negative dentry in each bucket unless they were referenced.
>>
>> At first dput of negative dentry when it swept to the tail of siblings
>> we'll also clear it's reference flag and look at next dentries in chain.
>> Then kill third in series of negative, unused and unreferenced denries.
>>
>> This way each hash bucket will preserve three negative dentry to let them
>> get reference and survive. Adding positive or used dentry into hash chain
>> also protects few recent negative dentries. In result total size of dcache
>> asymptotically limited by count of buckets and positive or used dentries.
>>
>> This heuristic isn't bulletproof and solves only most practical case.
>> It's easy to deceive: just touch same random name twice.
> 
> I'm not sure if that's "easy to deceive" ... My concern with limiting
> negative dentries is something like a kernel compilation where there
> are many (11 for mm/mmap.c, 9 in general) and there will be a lot of
> places where <linux/fs.h> does not exist
> 
> -isystem /usr/lib/gcc/x86_64-linux-gnu/9/include
> -I../arch/x86/include
> -I./arch/x86/include/generated
> -I../include
> -I./include
> -I../arch/x86/include/uapi
> -I./arch/x86/include/generated/uapi
> -I../include/uapi
> -I./include/generated/uapi
> -I ../mm
> -I ./mm
> 
> So it'd be good to know that kernel compilation times are unaffected by
> this patch.
> 

It's very unlikely that this patches changes anything for compilation.
Or any other scenario with sane amount and rate of appearing new names.

This trims only dentries which never been accessed twice.
Keeping 3 dentries per bucket gives high chances that all of them get
reference bit and stay in cache until shrinker bury them.

To get false positive in this heuristic - multiple newly created
negative dentries must hit one bucket in short period of time.
I.e. at least three hash collisions is required.