linux-kernel - Re: [PATCH v2 2/2] mm: collect the number of anon large folios on split

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Message-ID: <6c01b794-7c51-4d90-a215-210ac21401d2@redhat.com>
Date: Wed, 21 Aug 2024 23:39:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: baolin.wang@...ux.alibaba.com, chrisl@...nel.org, hanchuanhua@...o.com,
 ioworker0@...il.com, kaleshsingh@...gle.com, kasong@...cent.com,
 linux-kernel@...r.kernel.org, ryan.roberts@....com, v-songbaohua@...o.com,
 ziy@...dia.com, yuanshuai@...o.com, Usama Arif <usamaarif642@...il.com>
Subject: Re: [PATCH v2 2/2] mm: collect the number of anon large folios on
 split_deferred list

On 12.08.24 00:49, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> When an mTHP is added to the deferred_list, its partial pages
> are unused, leading to wasted memory and potentially increasing
> memory reclamation pressure.
> 
> Detailing the specifics of how unmapping occurs is quite difficult
> and not that useful, so we adopt a simple approach: each time an
> mTHP enters the deferred_list, we increment the count by 1; whenever
> it leaves for any reason, we decrement the count by 1.
> 
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>   Documentation/admin-guide/mm/transhuge.rst | 5 +++++
>   include/linux/huge_mm.h                    | 1 +
>   mm/huge_memory.c                           | 6 ++++++
>   3 files changed, 12 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 9fdfb46e4560..7072469de8a8 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -532,6 +532,11 @@ nr_anon
>          These huge pages could be entirely mapped or have partially
>          unmapped/unused subpages.
>   
> +nr_split_deferred
> +       the number of anon huge pages which have been partially unmapped
> +       and put onto split queue. Those unmapped subpages are also unused
> +       and temporarily wasting memory.

The name suggests something else ... like a counter of how many have 
been deferred split :)

Would "nr_anon_partially_mapped" "nr_anon_split_pending" (or something 
less mouthful) be clearer?

(likely "anon" really should be part of the name in any case)

The name we chose (and the implied semantics) will likely have 
implications on the handling of Usamas series.

-- 
Cheers,

David / dhildenb