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

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Message-ID: <ZML22YJi5vPBDEDj@dread.disaster.area>
Date:   Fri, 28 Jul 2023 08:59:37 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Damien Le Moal <dlemoal@...nel.org>
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 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....

-Dave.
-- 
Dave Chinner
david@...morbit.com