linux-kernel - Re: [PATCH v3 2/2] mm: collect the number of anon large folios partially mapped

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Message-ID: <23b3d29f-b285-4cc0-947e-49a555042ade@redhat.com>
Date: Fri, 23 Aug 2024 13:37:15 +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,
 yuanshuai@...o.com, ziy@...dia.com, usamaarif642@...il.com
Subject: Re: [PATCH v3 2/2] mm: collect the number of anon large folios
 partially mapped

On 23.08.24 00:40, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> When an mTHP is added to the deferred_list due to partially mapped,
> 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.

:) And here you only talk about mTHP but not in the subject.

> 
> 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 b78f2148b242..b1c948c7de9d 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -556,6 +556,11 @@ nr_anon
>          These huge pages could be entirely mapped or have partially
>          unmapped/unused subpages.
>   
> +nr_anon_partially_mapped
> +       the number of transparent anon huge pages which have been partially
> +       unmapped and put onto split queue. Those unmapped subpages are
> +       also unused and temporarily wasting memory.

It's a bit more tricky I believe:

"the number of anonymous THP which are likely partially mapped, possibly 
wasting memory, and have been queued for deferred memory reclamation. 
Note that in corner some cases (e.g., failed migration), we might detect 
an anonymous THP as "partially mapped" and count it here, even though it 
is not actually partially mapped anymore."

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb