linux-kernel - Re: [PATCH 2/2] mm: add docs for per-order mTHP split counters

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Date: Wed, 24 Apr 2024 16:34:49 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: 21cnbao@...il.com, david@...hat.com, baolin.wang@...ux.alibaba.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm: add docs for per-order mTHP split counters

On 24/04/2024 14:51, Lance Yang wrote:
> This commit introduces documentation for mTHP split counters in
> transhuge.rst.
> 
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index f82300b9193f..35d574a531c8 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -475,6 +475,22 @@ anon_swpout_fallback
>  	Usually because failed to allocate some continuous swap space
>  	for the huge page.
>  
> +split_page
> +	is incremented every time a huge page is split into base

perhaps "...successfully split into base..." to make it clear that this is only
incremented on success.

> +	pages. This can happen for a variety of reasons but a common
> +	reason is that a huge page is old and is being reclaimed.
> +	This action implies splitting all PMD/PTE mapped with the huge page.

What does it mean to "split all PTE"? It's already at its smallest granularity.
Perhaps "This action implies splitting any block mappings into PTEs."?

> +
> +split_page_failed
> +	is incremented if kernel fails to split huge
> +	page. This can happen if the page was pinned by somebody.
> +
> +deferred_split_page
> +	is incremented when a huge page is put onto split
> +	queue. This happens when a huge page is partially unmapped and
> +	splitting it would free up some memory. Pages on split queue are
> +	going to be split under memory pressure.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help