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From: "Huang, Ying" <ying.huang@intel.com>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,  David Hildenbrand
 <david@redhat.com>,  Matthew Wilcox <willy@infradead.org>,  Gao Xiang
 <xiang@kernel.org>,  Yu Zhao <yuzhao@google.com>,  Yang Shi
 <shy828301@gmail.com>,  Michal Hocko <mhocko@suse.com>,  Kefeng Wang
 <wangkefeng.wang@huawei.com>,  Barry Song <21cnbao@gmail.com>,  Chris Li
 <chrisl@kernel.org>,  <linux-mm@kvack.org>,
  <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 3/6] mm: swap: Simplify struct percpu_cluster
In-Reply-To: <20240311150058.1122862-4-ryan.roberts@arm.com> (Ryan Roberts's
	message of "Mon, 11 Mar 2024 15:00:55 +0000")
References: <20240311150058.1122862-1-ryan.roberts@arm.com>
	<20240311150058.1122862-4-ryan.roberts@arm.com>
Date: Tue, 12 Mar 2024 15:52:08 +0800
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Ryan Roberts <ryan.roberts@arm.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@arm.com>

LGTM, Thanks!

--
Best Regards,
Huang, Ying