linux-kernel - Re: [RFC v2] mm: introduce page pin owner

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Date:   Fri, 14 Jan 2022 19:47:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        John Dias <joaodias@...gle.com>, huww98@...look.com,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC v2] mm: introduce page pin owner


>>>>>> Otherwise, I'd like to have feature naming more higher level>>>>>> to represent page migration failure and then tracking unref of
>>>>>> the page. In the sense, PagePinOwner John suggested was good
>>>>>> candidate(Even, my original naming PagePinner was worse) since
>>>>>
>>>>> Personally, I dislike both variants.
>>>>>
>>>>>> I was trouble to abstract the feature with short word.
>>>>>> If we approach "what feature is doing" rather than "what's
>>>>>> the feature's goal"(I feel the your suggestion would be close
>>>>>> to what feature is doing), I'd like to express "unreference on
>>>>>> migraiton failed page" so PAGE_EXT_UNMIGRATED_UNREF
>>>>>> (However, I prefer the feature naming more "what we want to achieve")
>>>>>>
>>>>> E.g., PAGE_EXT_TRACE_UNREF will trace unref to the page once the bit is
>>>>> set. The functionality itself is completely independent of migration
>>>>> failures. That's just the code that sets it to enable the underlying
>>>>> tracing for that specific page.
>>>>
>>>> I agree that make something general is great but I also want to avoid
>>>> create something too big from the beginning with just imagination.
>>>> So, I'd like to hear more concrete and appealing usecases and then
>>>> we could think over this trace approach is really the best one to
>>>> achieve the goal. Once it's agreed, the naming you suggested would
>>>> make sense. 
>>>
>>> At least for me it's a lot cleaner if a feature clearly expresses what
>>> it actually does. Staring at PAGE_EXT_PIN_OWNER I initially had no clue.
>>> I was assuming we would actually track (not trace!) all active FOLL_PIN
>>> (not unref callers!). Maybe that makes it clearer why I'd prefer a
>>> clearer name.
>>
>> I totally agree PagePinOwner is not 100% straightforward. I'm open for
>> other better name. Currently we are discussing how we could generalize
>> and whether it's useful or not. Depending on the discussion, the design/
>> interface as well as naming could be changed. No problem.
> 
> PagePinOwner is just highly misleading. Because that's not what the
> feature does. Having that said, i hope we'll get other opinions as well.

FWIW, I think "page reference holder" would be clearer. PageRefHolder or
PageReferenceHolder

"Trace page reference holders on unref after migration of a page failed."

-- 
Thanks,

David / dhildenb