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

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Date: Thu, 27 Jul 2023 19:20:46 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: 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,
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 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,
 david@...morbit.com, tkhai@...ru, vbabka@...e.cz, roman.gushchin@...ux.dev,
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 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 7/27/23 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... So not adding that super simple
helper is not exactly the best choice in my opinion.

-- 
Damien Le Moal
Western Digital Research