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

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Message-ID: <33d6aec8-b4fc-aa37-27f4-f33984ea33d3@redhat.com>
Date:   Tue, 31 May 2022 14:58:21 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.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 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?


-- 
Thanks,

David / dhildenb