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

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Date: Tue, 12 Mar 2024 15:52:08 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Ryan Roberts <ryan.roberts@....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

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!

--
Best Regards,
Huang, Ying