linux-kernel - Re: [PATCH 6/9] mm/z3fold: move decrement of pool->pages_nr into __release_z3fold

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Date:   Thu, 3 Mar 2022 10:27:14 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Vitaly Wool <vitaly.wool@...sulko.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH 6/9] mm/z3fold: move decrement of pool->pages_nr into
 __release_z3fold_page()

On 2022/3/2 18:19, Vitaly Wool wrote:
> On Wed, Mar 2, 2022 at 10:12 AM David Laight <David.Laight@...lab.com> wrote:
>>
>>>> Atomic operations aren't magic.
>>>> Atomic operations are (at best) one slow locked bus cycle.
>>>> Acquiring a lock is the same.
>>>> Releasing a lock might be cheaper, but is probably a locked bus cycle.
>>>>
>>>> So if you use state_lock to protect pages_nr then you lose an atomic
>>>> operation for the decrement and gain one (for the unlock) in the increment.
>>>> That is even or maybe a slight gain.
>>>> OTOH a 64bit atomic is a PITA on some 32bit systems.
>>>> (In fact any atomic is a PITA on sparc32.)
>>>
>>> It's actually *stale_lock* and it's very misleading to use it for this.
>>> I would actually like to keep atomics but I have no problem with
>>> making it 32-bit for 32-bit systems. Would that work for you guys?
>>
>> It would be better to rename the lock.
> 
> No it would not because that lock is protecting the list of entries
> that could not be immediately freed.
> 

Or could we use pool->lock to do this ?

> ~Vitaly

Vitaly, is the patch itself worth a Reviewed-by tag and go to the mm-tree ? Could this
enhance discussed here be sent as another separate patch or am I supposed to make this
change into the current patch?

Many thanks for comment.

> .
>