linux-kernel - Re: [PATCH v4 3/6] mm: swap: Simplify struct percpu

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Message-ID: <2e236d6b-e6fc-428a-ad06-c2fb1dbf5c8b@arm.com>
Date: Tue, 12 Mar 2024 08:51:23 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>,
 Gao Xiang <xiang@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
 Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/6] mm: swap: Simplify struct percpu_cluster

On 12/03/2024 07:52, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@....com> writes:
> 
>> struct percpu_cluster stores the index of cpu's current cluster and the
>> offset of the next entry that will be allocated for the cpu. These two
>> pieces of information are redundant because the cluster index is just
>> (offset / SWAPFILE_CLUSTER). The only reason for explicitly keeping the
>> cluster index is because the structure used for it also has a flag to
>> indicate "no cluster". However this data structure also contains a spin
>> lock, which is never used in this context, as a side effect the code
>> copies the spinlock_t structure, which is questionable coding practice
>> in my view.
>>
>> So let's clean this up and store only the next offset, and use a
>> sentinal value (SWAP_NEXT_INVALID) to indicate "no cluster".
>> SWAP_NEXT_INVALID is chosen to be 0, because 0 will never be seen
>> legitimately; The first page in the swap file is the swap header, which
>> is always marked bad to prevent it from being allocated as an entry.
>> This also prevents the cluster to which it belongs being marked free, so
>> it will never appear on the free list.
>>
>> This change saves 16 bytes per cpu. And given we are shortly going to
>> extend this mechanism to be per-cpu-AND-per-order, we will end up saving
>> 16 * 9 = 144 bytes per cpu, which adds up if you have 256 cpus in the
>> system.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> 
> LGTM, Thanks!

Thanks! What's a guy got to do to get Rb or Ack? :)

> 
> --
> Best Regards,
> Huang, Ying
>