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Message-ID: <d108bd79-086b-4564-838b-d41afa055137@redhat.com>
Date: Thu, 22 Feb 2024 11:19:21 +0100
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Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags
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To: Ryan Roberts <ryan.roberts@arm.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Huang Ying <ying.huang@intel.com>,
 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>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20231025144546.577640-1-ryan.roberts@arm.com>
 <20231025144546.577640-2-ryan.roberts@arm.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <20231025144546.577640-2-ryan.roberts@arm.com>
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On 25.10.23 16:45, 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
> 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.

That is the  __try_to_reclaim_swap() -> folio_free_swap() -> 
folio_swapped() -> swap_page_trans_huge_swapped() call chain I assume.

The "difference" is that you will now (1) get another temporary 
reference on the folio and (2) (try)lock the folio every time you 
discard a single PTE of a (possibly) large THP.

-- 
Cheers,

David / dhildenb