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Date: Fri, 14 Jun 2024 18:06:06 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>, Ryan Roberts <ryan.roberts@arm.com>,
 "Huang, Ying" <ying.huang@intel.com>, Kalesh Singh
 <kaleshsingh@google.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
 Barry Song <baohua@kernel.org>
Subject: Re: [PATCH v2 0/2] mm: swap: mTHP swap allocator base on swap
 cluster order
Message-Id: <20240614180606.5f3b6f4a6cd515df30b7a0e4@linux-foundation.org>
In-Reply-To: <20240614-swap-allocator-v2-0-2a513b4a7f2f@kernel.org>
References: <20240614-swap-allocator-v2-0-2a513b4a7f2f@kernel.org>
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On Fri, 14 Jun 2024 16:48:06 -0700 Chris Li <chrisl@kernel.org> wrote:

> This is the short term solutiolns "swap cluster order" listed
> in my "Swap Abstraction" discussion slice 8 in the recent
> LSF/MM conference.
> 
> When commit 845982eb264bc "mm: swap: allow storage of all mTHP
> orders" is introduced, it only allocates the mTHP swap entries
> from new empty cluster list.  It has a fragmentation issue
> reported by Barry.
> 
> https://lore.kernel.org/all/CAGsJ_4zAcJkuW016Cfi6wicRr8N9X+GJJhgMQdSMp+Ah+NSgNQ@mail.gmail.com/
> 
> The mTHP allocation failure rate raises to almost 100% after a few
> hours in Barry's test run.
> 
> The reason is that all the empty cluster has been exhausted while
> there are planty of free swap entries to in the cluster that is
> not 100% free.
> 
> Remember the swap allocation order in the cluster.
> Keep track of the per order non full cluster list for later allocation.
> 
> This greatly improve the sucess rate of the mTHP swap allocation.
> 

I'm having trouble understanding the overall impact of this on users. 
We fail the mTHP swap allocation and fall back, but things continue to
operate OK?

> There is some test number in the V1 thread of this series:
> https://lore.kernel.org/r/20240524-swap-allocator-v1-0-47861b423b26@kernel.org

Well, please let's get the latest numbers into the latest patchset. 
Along with a higher-level (and quantitative) description of the user impact.

I'll add this into mm-unstable now for some exposure, but at this point
I'm not able to determine whether it should go in as a hotfix for
6.10-rcX.