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Date: Wed, 28 Feb 2024 13:12:21 +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>
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From: David Hildenbrand <david@redhat.com>
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>> How relevant is it? Relevant enough that someone decided to put that
>> optimization in? I don't know :)
> 
> I'll have one last go at convincing you: Huang Ying (original author) commented
> "I believe this should be OK.  Better to compare the performance too." at [1].
> That implies to me that perhaps the optimization wasn't in response to a
> specific problem after all. Do you have any thoughts, Huang?

Might make sense to include that in the patch description!

> OK so if we really do need to keep this optimization, here are some ideas:
> 
> Fundamentally, we would like to be able to figure out the size of the swap slot
> from the swap entry. Today swap supports 2 sizes; PAGE_SIZE and PMD_SIZE. For
> PMD_SIZE, it always uses a full cluster, so can easily add a flag to the cluster
> to mark it as PMD_SIZE.
> 
> Going forwards, we want to support all sizes (power-of-2). Most of the time, a
> cluster will contain only one size of THPs, but this is not the case when a THP
> in the swapcache gets split or when an order-0 slot gets stolen. We expect these
> cases to be rare.
> 
> 1) Keep the size of the smallest swap entry in the cluster header. Most of the
> time it will be the full size of the swap entry, but sometimes it will cover
> only a portion. In the latter case you may see a false negative for
> swap_page_trans_huge_swapped() meaning we take the slow path, but that is rare.
> There is one wrinkle: currently the HUGE flag is cleared in put_swap_folio(). We
> wouldn't want to do the equivalent in the new scheme (i.e. set the whole cluster
> to order-0). I think that is safe, but haven't completely convinced myself yet.
> 
> 2) allocate 4 bits per (small) swap slot to hold the order. This will give
> precise information and is conceptually simpler to understand, but will cost
> more memory (half as much as the initial swap_map[] again).
> 
> I still prefer to avoid this at all if we can (and would like to hear Huang's
> thoughts). But if its a choice between 1 and 2, I prefer 1 - I'll do some
> prototyping.

Taking a step back: what about we simply batch unmapping of swap entries?

That is, if we're unmapping a PTE range, we'll collect swap entries 
(under PT lock) that reference consecutive swap offsets in the same swap 
file.

There, we can then first decrement all the swap counts, and then try 
minimizing how often we actually have to try reclaiming swap space 
(lookup folio, see it's a large folio that we cannot reclaim or could 
reclaim, ...).

Might need some fine-tuning in swap code to "advance" to the next entry 
to try freeing up, but we certainly can do better than what we would do 
right now.

-- 
Cheers,

David / dhildenb