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From: "Huang, Ying" <ying.huang@intel.com>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Kairui Song <ryncsn@gmail.com>,  <linux-mm@kvack.org>,  Chris Li
 <chrisl@kernel.org>,  Minchan Kim <minchan@kernel.org>,  Barry Song
 <v-songbaohua@oppo.com>,  Yu Zhao <yuzhao@google.com>,  SeongJae Park
 <sj@kernel.org>,  David Hildenbrand <david@redhat.com>,  Yosry Ahmed
 <yosryahmed@google.com>,  Johannes Weiner <hannes@cmpxchg.org>,  Matthew
 Wilcox <willy@infradead.org>,  Nhat Pham <nphamcs@gmail.com>,  Chengming
 Zhou <zhouchengming@bytedance.com>,  "Andrew Morton"
 <akpm@linux-foundation.org>,  <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 00/10] mm/swap: always use swap cache for
 synchronization
In-Reply-To: <58e4f0c2-99d1-42b9-ab70-907cf35ac1a7@arm.com> (Ryan Roberts's
	message of "Wed, 27 Mar 2024 08:27:34 +0000")
References: <20240326185032.72159-1-ryncsn@gmail.com>
	<878r24o07p.fsf@yhuang6-desk2.ccr.corp.intel.com>
	<CAMgjq7B4Aejv16XK-zm9zPFrtVV1YutYWk5Ywzy8GwfMsTAXgA@mail.gmail.com>
	<58e4f0c2-99d1-42b9-ab70-907cf35ac1a7@arm.com>
Date: Wed, 27 Mar 2024 16:32:31 +0800
Message-ID: <87a5mkm5wg.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Ryan Roberts <ryan.roberts@arm.com> writes:

> [...]
>
>>>> Test 1, sequential swapin/out of 30G zero page on ZRAM:
>>>>
>>>>                Before (us)        After (us)
>>>> Swapout:       33619409           33886008
>>>> Swapin:        32393771           32465441 (- 0.2%)
>>>> Swapout (THP): 7817909            6899938  (+11.8%)
>>>> Swapin (THP) : 32452387           33193479 (- 2.2%)
>>>
>>> If my understanding were correct, we don't have swapin (THP) support,
>>> yet.  Right?
>> 
>> Yes, this series doesn't change how swapin/swapout works with THP in
>> general, but now THP swapout will leave shadows with large order, so
>> it needs to be splitted upon swapin, that will slow down later swapin
>> by a little bit but I think that's worth it.
>> 
>> If we can do THP swapin in the future, this split on swapin can be
>> saved to make the performance even better.
>
> I'm confused by this (clearly my understanding of how this works is incorrect).
> Perhaps you can help me understand:
>
> When you talk about "shadows" I assume you are referring to the swap cache? It
> was my understanding that swapping out a THP would always leave the large folio
> in the swap cache, so this is nothing new?
>
> And on swap-in, if the target page is in the swap cache, even if part of a large
> folio, why does it need to be split? I assumed the single page would just be
> mapped? (and if all the other pages subsequently fault, then you end up with a
> fully mapped large folio back in the process)?
>
> Perhaps I'm misunderstanding what "shadows" are?

Perhaps, shadow is used to support workingset protection/detection on
the anonymous LRU list as in the following patchset (merged).

https://lore.kernel.org/all/1595490560-15117-5-git-send-email-iamjoonsoo.kim@lge.com/T/#m962395eb5968c74b0c4c8e41d4b0dcdd3f28b2e6

--
Best Regards,
Huang, Ying