linux-kernel - Re: [PATCH v6 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster

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Message-ID: <af32865e-1362-48f0-a2f1-d6889b685203@redhat.com>
Date: Fri, 5 Apr 2024 11:25:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>,
 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>, Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags

On 03.04.24 13:40, Ryan Roberts wrote:
> 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
> shortcut a 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 functionally equivalent.
> 
> That said, removing the fast path means that we will take a reference
> and trylock a large folio much more often, which we would like to avoid.
> The next patch will solve this.
> 
> 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.
> 
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---

[started to review v5, then saw v6]

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb