linux-kernel - Re: [PATCH 2/3] mm/swapfile: avoid confusing swap cache statistics

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Message-ID: <c7d0d606-61ba-9cfc-3c2a-961a88b6c3e5@huawei.com>
Date:   Wed, 1 Jun 2022 10:11:16 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] mm/swapfile: avoid confusing swap cache statistics

On 2022/5/31 20:58, David Hildenbrand wrote:
> On 31.05.22 04:55, Miaohe Lin wrote:
>> On 2022/5/31 7:04, Andrew Morton wrote:
>>> On Fri, 27 May 2022 17:26:25 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:
>>>
>>>> At swapoff time, we're going to swap in the pages continuously. So calling
>>>> lookup_swap_cache would confuse statistics. We should use find_get_page
>>>> directly here.
>>>
>>> Why is the existing behaviour wrong?  swapoff() has to swap stuff in to
>>> be able to release the swap device.  Why do you believe that this
>>> swapin activity should not be accounted?
>>
>> IMHO, statistics, e.g. swap_cache_info.find_success, are used to show the effectiveness
>> of the swap cache activity. So they should only reflect the memory accessing activity
>> of the user. I think swapoff can't reflect the effectiveness of the swap cache activity
>> because it just swaps in pages one by one. Or statistics should reflect all the activity
>> of the user including swapoff?
> 
> I'm wondering who cares and why?

I thought it's used to show the effectiveness of the swapcache readahead algorithm. If nobody
ever cares about it now, I'm fine to drop this patch. And could these statistics be removed
since nobody cares about it?

Thanks!

> 
>