Virtualization - Jul 2023 - [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 at fromorbit.com