linux-kernel - Re: [RFC PATCH v1 1/2] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster

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Date:   Wed, 11 Oct 2023 15:43:36 +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>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [RFC PATCH v1 1/2] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags

Ryan Roberts <ryan.roberts@....com> writes:

> As preparation for supporting small-sized THP in the swap-out path,
> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
> which, when present, always implies PMD-sized THP, which is the same as
> the cluster size.
>
> The only use of the flag was to determine whether a swap entry refers to
> a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
> Instead of relying on the flag, we now pass in nr_pages, which
> originates from the folio's number of pages. This allows the logic to
> work for folios of any order.
>
> The one snag is that one of the swap_page_trans_huge_swapped() call
> sites does not have the folio. But it was only being called there to
> avoid bothering to call __try_to_reclaim_swap() in some cases.
> __try_to_reclaim_swap() gets the folio and (via some other functions)
> calls swap_page_trans_huge_swapped(). So I've removed the problematic
> call site and believe the new logic should be equivalent.

I believe this should be OK.  Better to compare the performance too.

> Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster()
> which used to be called during folio splitting, since
> split_swap_cluster()'s only job was to remove the flag.
>

--
Best Regards,
Huang, Ying