linux-kernel - Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster

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Date: Thu, 22 Feb 2024 11:20:48 +0100
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>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags

On 22.02.24 11:19, David Hildenbrand wrote:
> 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.
> 

Thinking about it, your change will not only affect THP, but any call to 
free_swap_and_cache().

Likely that's not what we want. :/

-- 
Cheers,

David / dhildenb