netdev - Re: [PATCH v3 28/49] dm zoned: dynamically allocate the dm-zoned-meta shrinker

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Date: Fri, 28 Jul 2023 08:48:37 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Dave Chinner <david@...morbit.com>
Cc: Qi Zheng <zhengqi.arch@...edance.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, x86@...nel.org, kvm@...r.kernel.org,
 xen-devel@...ts.xenproject.org, linux-erofs@...ts.ozlabs.org,
 linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
 linux-nfs@...r.kernel.org, linux-mtd@...ts.infradead.org,
 rcu@...r.kernel.org, netdev@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
 dm-devel@...hat.com, linux-raid@...r.kernel.org,
 linux-bcache@...r.kernel.org, virtualization@...ts.linux-foundation.org,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
 Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
 tkhai@...ru, vbabka@...e.cz, roman.gushchin@...ux.dev, djwong@...nel.org,
 brauner@...nel.org, paulmck@...nel.org, tytso@....edu, steven.price@....com,
 cel@...nel.org, senozhatsky@...omium.org, yujie.liu@...el.com,
 gregkh@...uxfoundation.org, muchun.song@...ux.dev
Subject: Re: [PATCH v3 28/49] dm zoned: dynamically allocate the dm-zoned-meta
 shrinker

On 7/28/23 07:59, Dave Chinner wrote:
> On Thu, Jul 27, 2023 at 07:20:46PM +0900, Damien Le Moal wrote:
>> On 7/27/23 17:55, Qi Zheng wrote:
>>>>>           goto err;
>>>>>       }
>>>>>   +    zmd->mblk_shrinker->count_objects = dmz_mblock_shrinker_count;
>>>>> +    zmd->mblk_shrinker->scan_objects = dmz_mblock_shrinker_scan;
>>>>> +    zmd->mblk_shrinker->seeks = DEFAULT_SEEKS;
>>>>> +    zmd->mblk_shrinker->private_data = zmd;
>>>>> +
>>>>> +    shrinker_register(zmd->mblk_shrinker);
>>>>
>>>> I fail to see how this new shrinker API is better... Why isn't there a
>>>> shrinker_alloc_and_register() function ? That would avoid adding all this code
>>>> all over the place as the new API call would be very similar to the current
>>>> shrinker_register() call with static allocation.
>>>
>>> In some registration scenarios, memory needs to be allocated in advance.
>>> So we continue to use the previous prealloc/register_prepared()
>>> algorithm. The shrinker_alloc_and_register() is just a helper function
>>> that combines the two, and this increases the number of APIs that
>>> shrinker exposes to the outside, so I choose not to add this helper.
>>
>> And that results in more code in many places instead of less code + a simple
>> inline helper in the shrinker header file...
> 
> It's not just a "simple helper" - it's a function that has to take 6
> or 7 parameters with a return value that must be checked and
> handled.
> 
> This was done in the first versions of the patch set - the amount of
> code in each caller does not go down and, IMO, was much harder to
> read and determine "this is obviously correct" that what we have
> now.
> 
>> So not adding that super simple
>> helper is not exactly the best choice in my opinion.
> 
> Each to their own - I much prefer the existing style/API over having
> to go look up a helper function every time I want to check some
> random shrinker has been set up correctly....

OK. All fair points.


-- 
Damien Le Moal
Western Digital Research