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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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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
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Date:   Fri, 8 May 2020 16:00:08 -0400
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On 5/8/20 3:38 PM, Konstantin Khlebnikov wrote:
>
>
> On 08/05/2020 22.05, Waiman Long wrote:
>> 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?
>
> I see no problem in adding more numbers into sysctl.
> Especially into such rarely used.
> This interface is designed for that.
>
> Also fields 'age_limit' and 'want_pages' are unused since kernel 2.2.0

Well, I got rebuke the last time I want to reuse one of 
age_limit/want_pages entry for negative dentry count because of the 
potential of breaking some really old applications or tools. Changing 
dentry-state to output one more number can potentially break 
compatibility too. That is why I am questioning if it is a good idea to 
use up the last free slot.

Cheers,
Longman