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Subject: Re: [PATCH RFC 1/8] dcache: show count of hash buckets in sysctl
 fs.dentry-state
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>
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 <7c1cef87-2940-eb17-51d4-cbc40218b770@redhat.com>
 <ac1ece33-46ea-175a-98ef-c79fcd1ced90@yandex-team.ru>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <741172f7-a0d2-1428-fb25-789e38978d4e@redhat.com>
Date:   Fri, 8 May 2020 15:05:18 -0400
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On 5/8/20 12:16 PM, Konstantin Khlebnikov wrote:
> On 08/05/2020 17.49, Waiman Long wrote:
>> On 5/8/20 8:23 AM, Konstantin Khlebnikov wrote:
>>> Count of buckets is required for estimating average length of hash 
>>> chains.
>>> Size of hash table depends on memory size and printed once at boot.
>>>
>>> Let's expose nr_buckets as sixth number in sysctl fs.dentry-state
>>
>> The hash bucket count is a constant determined at boot time. Is there 
>> a need to use up one dentry_stat entry for that? Besides one can get 
>> it by looking up the kernel dmesg log like:
>>
>> [    0.055212] Dentry cache hash table entries: 8388608 (order: 14, 
>> 67108864 bytes)
>
> Grepping logs since boot time is a worst API ever.
>
> dentry-state shows count of dentries in various states.
> It's very convenient to show count of buckets next to it,
> because this number defines overall scale. 

I am not against using the last free entry for that. My only concern is 
when we want to expose another internal dcache data point via 
dentry-state, we will have to add one more number to the array which can 
cause all sort of compatibility problem. So do we want to use the last 
free slot for a constant that can be retrieved from somewhere else?

Cheers,
Longman